A source document is the written record a transaction produces at the moment it happens, and it is the evidence a bookkeeping entry is made from. It comes from outside the books, because the sale, the purchase, the payroll run or the payment generates it, so it exists before anyone records anything. An invoice, a receipt, a deposit slip and a canceled check are all source documents.
What a Source Document Has to Show
A document is evidence rather than paper when it carries four facts: the amount, the date, the other party, and the business purpose.
Three of those the IRS states directly. Supporting documents for gross receipts have to show the amounts and the sources of those receipts, and supporting documents for expenses have to show the amount paid and that the amount was for a business expense (IRS Publication 583, Starting a Business and Keeping Records, Rev. December 2024).
The date is what assigns the item to a period, which is why the instruction for keeping these documents is to organize them by year and by type of income or expense (IRS Publication 583, Rev. December 2024). A dateless document evidences that something happened, not when.
What can stand in when the document itself is gone is narrow, and so is what a proof of payment establishes on its own. Both are worked through in canceled check.
The Documents a Firm Actually Handles
Purchases, sales, payroll and other business transactions generate supporting documents, and the IRS names the category directly: sales slips, paid bills, invoices, receipts, deposit slips and canceled checks (IRS Publication 583, Rev. December 2024).
Which one you reach for depends on what you are proving. Gross receipts are shown by cash register tapes, bank deposit slips, receipt books, invoices, credit card charge slips, and Forms 1099-MISC and 1099-NEC, the information returns a payer files for amounts it paid out. Inventory cost is shown by canceled checks, cash register tape receipts, credit card sales slips and invoices, and other expenses by those plus account statements and petty cash slips for small cash payments (IRS Publication 583, Rev. December 2024).
Notice where the canceled check lands. It sits on the inventory and expense lists and not on the gross receipts list, because it proves that a payment cleared rather than what the payment bought.
What the Law Requires, and What the Document Adds
The law requires records, not source documents. Section 6001 of the Internal Revenue Code puts the recordkeeping duty on every person liable for any tax imposed by the title, or for the collection of it (26 U.S.C. 6001). The regulation under it names the record as permanent books of account or records, including inventories, sufficient to establish the amount of gross income, deductions, credits, or other matters a return has to show (eCFR, 26 CFR 1.6001-1(a)). Both are laid out in bookkeeping.
Neither authority uses the phrase “source document”. What the document does is make those books sufficient, and it is kept for that stated reason, that it supports the entries in the books and on the tax return (IRS Publication 583, Rev. December 2024). A set of books can balance perfectly and establish nothing.
Source Document Versus the Record Made From It
The document and the entry get treated as one thing and they are two. The source document is external evidence created by the transaction itself. The journal entry is the internal record a bookkeeper writes from that document afterwards, in the firm’s own accounts. One is what happened, the other is what was recorded about it.
Keeping the link between the two is what an audit trail is, and the document is where the accounting cycle starts.
Common Questions
To stand behind the entry. These documents are kept because they support the entries in your books and on your tax return (IRS Publication 583, Rev. December 2024). An entry with nothing behind it is an assertion.
Invoices, receipts, deposit slips, canceled checks and cash register tapes. All five sit in the IRS’s own lists of supporting documents (IRS Publication 583, Rev. December 2024).
If that document-to-entry work is stacking up in your firm, a trial task puts a block of your own work in front of your own reviewer before any client file is committed.
