Most people meet the phrase for the first time in an email from their accountant: "can you send me the source documents for these?" It sounds like a technical category. It is actually the simplest idea in accounting — the piece of paper, or the PDF, that proves the transaction was real.
Quick answer: what is a source document in accounting?
A source document is the original record evidencing a business transaction — who, what, when and how much — from which accounting entries are made. Receipts, supplier and customer invoices, bank statements, purchase orders, contracts and payroll records are the usual examples. Every journal entry should trace back to one, which is why source documents are the foundation of the audit trail and of any deduction you later have to substantiate.
What is a source document?
A source document is the original record that evidences a business transaction. Corporate Finance Institute's glossary puts it as the paper trail a financial transaction generates, and notes that these documents generally carry the date, the total amount, a description and one or more authorising signatures. That list of fields is the useful part: a document earns the name by recording who was involved, what was exchanged, when, and for how much.
The distinction worth holding on to is between the document and the entry. The source document is created at the moment of the transaction, usually by the other party — the vendor writes the invoice, the bank produces the statement, the customer signs the contract. The journal entry is your own description of that transaction in debits and credits, written afterwards. One is evidence; the other is bookkeeping. An entry with no document behind it is an assertion.
Examples of source documents
The list below covers what turns up in a typical small-business file. The second column is the one that matters in practice — what each document actually proves, which is rarely the whole transaction on its own.
| Document | What it proves | Where it comes from |
|---|---|---|
| Receipt | That a purchase happened, what was bought, and that it was paid | The vendor, at the point of sale |
| Supplier invoice | That an amount is owed to a supplier, for a stated supply | The supplier, before payment |
| Sales invoice | That you billed a customer, for what, on what terms | You, issued outward |
| Bank or card statement | That money moved, on a date, for an amount — not what it was for | The bank or card issuer |
| Purchase order | That an order was authorised, at agreed quantities and prices | You, issued to the supplier |
| Credit note | That an earlier invoice was reduced or reversed, and why | The supplier (or you, on a sales credit) |
| Payroll records | Hours, gross pay, deductions and net pay per employee per period | Your payroll system or provider |
| Contract or lease | The terms behind a recurring or long-running obligation | Both parties, signed |
| Payment confirmation or cheque stub | That a specific payment was made against a specific obligation | Your bank or payment provider |
| Deposit slip | That funds were paid in, and in what composition | Your bank |
| Mileage log | Date, distance, destination and business purpose of a trip | You, recorded as you drive |
The distinction that causes the most trouble: bank statement vs receipt
A bank statement proves payment. It shows that an amount left the account on a date, and names a counterparty as the bank recorded it — often a payment processor or a trading name that looks nothing like the business you bought from.
A receipt or invoice proves purpose. It shows what was actually bought and therefore whether it was a business cost at all. This is why "I have my bank statements" is not the same as "I have my records": the statement can substantiate the amount and date of a deduction while saying nothing about whether it qualifies.
Why source documents matter
They create the audit trail. The point of an audit trail is that any figure on a report can be walked backwards — report to ledger, ledger to journal, journal to document — until you reach something a third party created. Without the document, the trail ends at your own bookkeeping and stops being evidence. Our page on the expense audit trail covers what that chain looks like in practice.
They substantiate deductions. A deduction is a claim, and the document is what supports it if the return is examined. This is the reason retention rules exist at all: the tax authority's window to ask is longer than most people's filing habits.
They let you catch errors. Comparing the entry against the document is how transposed digits, duplicated invoices and miscoded categories surface. A monthly comparison is cheap; the same error found at year end has already flowed into a filed return. A regular monthly books review exists mostly for this.
They make fraud harder. When every entry has to match a document issued by someone outside the business, a fabricated expense requires a fabricated document. That is a materially higher bar than editing a spreadsheet cell, and it is why separation between the person who records and the person who holds the evidence is a standard control.
Source documents vs journals, ledgers and reports
Source documents sit at the very front of the accounting cycle. Everything downstream is a summary of them:
source document → journal entry → ledger account → trial balance → financial statements
- The source document is the evidence — external, created at the time.
- The journal records the transaction chronologically, in debits and credits, referencing the document.
- The ledger reorganises those entries by account, so you can see everything that hit Office Supplies.
- The trial balance checks that total debits equal total credits.
- The financial statements present the result.
Each step compresses. By the time a number reaches a profit and loss statement it represents hundreds of documents, and the only way back to what it actually consists of is the reference the journal entry kept. This is why bookkeepers care so much about document numbering: the reference is the thread.
How long to keep source documents
Retention is set by how long the tax authority can still ask, not by how long the documents stay useful to you.
| Country | The rule | Source |
|---|---|---|
| United States | Keep records that support an item of income or deduction until the period of limitations for that return runs out — generally three years, with named situations that run longer (including six years where unreported income exceeds 25% of gross income, and no limit for a fraudulent or unfiled return). Employment tax records have their own four-year minimum. | IRS Publication 583, Starting a Business and Keeping Records |
| Canada | Keep records and supporting documents for six years from the end of the last tax year they relate to. If a return is filed late, the six years run from the date of filing. Destroying them earlier requires CRA permission. | CRA, Keeping records |
Two practical consequences. First, the retention clock is per return, not per document — a receipt from January is kept because of the return it ends up supporting, so a single cut-off date for a whole year's file is the workable simplification. Second, "generally three years" in the US is a floor with several named exceptions above it, which is why many firms simply adopt the longer period rather than sorting documents by which exception might apply.
If your question is specifically about receipts rather than the full document set, our receipt organizer guide covers how long to keep receipts for taxes and how to store them. Estimates — confirm with your tax professional.
Digital source documents — what changes and what doesn't
What does not change is the evidentiary job. A scanned or emailed document is a source document on the same terms as the paper: it has to be a complete and accurate reproduction, it has to be legible, and you have to be able to produce it when it is asked for. A photo of a receipt with the total cropped off is not a source document in any medium.
What does change is where the risk sits. Paper degrades and gets lost in boxes; digital files survive perfectly and then become unreachable — an inbox nobody can log into, a departed bookkeeper's drive, a folder of files named IMG_4471. Retrievability is the part digital storage makes easy to neglect, and it is explicitly part of what the rules ask for.
The IRS addresses electronic storage systems in Revenue Procedure 97-22, and the CRA publishes its own guidance on electronic record keeping. Both are worth reading in the original rather than in summary if you are designing a firm's retention policy — this page names them so you know what to look for, and states only the general standard rather than their detailed requirements. Estimates — confirm with your tax professional.
Capturing source documents without chasing them
The practical difficulty is almost never understanding what a source document is. It is that the documents arrive scattered — some in email, some as photos on a phone, some in a supplier portal — and someone has to reunite them with the entries they support, usually weeks later.
ExpenseBot is built around closing that gap at capture. It reads receipts and invoices out of Gmail, accepts direct uploads and Google Drive folders, and extracts vendor, date and amount into a Google Sheet the client owns. It classifies documents by type before an accounting push — a receipt becomes a QuickBooks Online Purchase, a supplier invoice becomes a Bill — and the original document image is attached to that QuickBooks record at push time, in the Attachments section of the transaction, the same place it would sit if someone had uploaded it by hand.
For a bookkeeper that inverts the usual order of work: the source documents arrive already linked to the rows they support, so month-end is a review rather than a retrieval exercise. The Gmail receipt scanner covers the capture side, and the bookkeeper page covers the client-workflow side. If you are building the surrounding vocabulary, the companion piece on general and administrative expenses covers how the captured costs get categorised once they land.
Worth being clear about the boundary: this is spend capture, not a general ledger. ExpenseBot does not keep double-entry books, produce a trial balance, or file anything. It gets the evidence attached to the transaction and into the accounting system your books actually live in.
Frequently Asked Questions
What is a source document in accounting?
A source document is the original record that proves a transaction took place — a receipt, a supplier invoice, a bank statement, a signed contract. It records who, what, when and how much, and it is the evidence each entry in the books traces back to. The journal entry describes the transaction; the source document proves it happened.
What are examples of source documents?
Receipts, supplier invoices, sales invoices, bank and credit-card statements, purchase orders, credit notes, payroll records, contracts and leases, cheque stubs and payment confirmations, deposit slips, and mileage logs. Corporate Finance Institute's glossary lists cheques, invoices, receipts, credit memos, employee time cards, deposit slips and purchase orders among the common types.
Why are source documents important?
They create the audit trail that lets any number in the accounts be traced back to evidence, they substantiate deductions if a return is examined, they let you catch data-entry errors by comparing the entry against the document, and they make fraud harder because every figure has to match a document someone else issued.
Is a bank statement a source document?
Yes, but only for the payment. A bank statement proves that money moved, on a date, for an amount, to a named counterparty. It does not prove what was bought or why it was a business cost — that is what a receipt or invoice adds. In practice you generally want both: the statement evidences the payment, the receipt evidences the purpose.
How long should source documents be kept?
In the United States, IRS Publication 583 says to keep records supporting an item of income or deduction until the period of limitations for that return runs out — generally three years, with named situations that run longer. In Canada, the CRA's Keeping records guidance says six years from the end of the last tax year the records relate to. Estimates — confirm with your tax professional.
Does a scanned receipt count as a source document?
Yes, provided the image is a complete and accurate reproduction of the original and you can retrieve it when it is asked for. The IRS addresses electronic storage systems in Revenue Procedure 97-22. The medium does not change what the document has to prove — a legible scan of a receipt evidences the same transaction the paper did. Estimates — confirm with your tax professional.
What is the difference between a source document and a journal entry?
The source document is external evidence created at the moment of the transaction, usually by the other party — the vendor writes the invoice, the bank produces the statement. The journal entry is your own internal record describing that transaction in debits and credits. One is evidence, the other is bookkeeping, and the entry is only as reliable as the document behind it.
Estimates — confirm with your tax professional. Retention periods and electronic-record rules are summarised here from the named primary sources and vary by situation, entity type and jurisdiction.