Source Document in Accounting: Meaning and Examples

What Is a Source Document in Accounting?

A source document is an original record that provides evidence and details of a financial transaction. It tells the bookkeeper what occurred, when it occurred, the amount, the parties involved, and often the business purpose or authorization.

Examples include customer invoices, vendor bills, receipts, credit memos, contracts, deposit records, bank statements, payroll reports, and loan agreements. A source document is the starting evidence for an accounting entry, but it does not decide the correct category or tax treatment by itself.

Common source-document examples

Different transactions require different evidence:

What information should a source document include?

A useful document generally identifies:

• Date of the transaction.
• Seller, customer, employee, lender, or other party.
• Amount and currency.
• Goods or services purchased or sold.
• Quantity, price, tax, discounts, and payment terms when applicable.
• Invoice, receipt, check, or transaction number.
• Business purpose when it is not obvious.
• Evidence of approval, delivery, performance, or payment when relevant.

No single list fits every transaction. A restaurant receipt may support the amount but not the attendees and business purpose. A credit-card statement may prove payment but not what was purchased. A contract may establish an obligation but not prove that the service was completed.

How source documents enter the books

The typical flow is:

1. The business creates or receives the document.
2. Someone reviews it for accuracy, authorization, and business purpose.
3. The transaction is entered or imported into the accounting system.
4. It is categorized to the appropriate account or accounts.
5. The entry is matched to payment or collection activity.
6. Reconciliation compares the books with an independent statement.
7. The source document remains linked or indexed for later review.

This flow creates part of the audit trail. It also reduces duplicate entry. For example, a vendor bill entered into accounts payable should normally be matched to the later bank payment. Importing the payment as a second expense would overstate costs.

Example: revenue and a later credit

Coastal Design LLC bills a customer $10,000. The signed proposal and invoice support the original sale. The journal entry debits Accounts Receivable and credits Service Revenue for $10,000.

The customer identifies an agreed service issue, and the company approves a $1,000 credit. A credit memo documents the reason and links to the invoice. The credit reduces the receivable and is recorded as a return or allowance against revenue under the company's accounting policy. The original invoice and credit memo together explain why gross revenue includes $10,000 while net revenue includes $9,000.

If the company simply changes the invoice from $10,000 to $9,000 without preserving the credit history, the final balance may be right but the audit trail is weaker.

Source document versus proof of payment

These terms overlap but are not identical. An invoice shows what a vendor says is owed. A canceled check, card statement, or bank record shows payment. For some items, the business should retain both.

Proof of payment alone may not establish deductibility. Tax records should support the amount, date, and business purpose and meet any special substantiation rules. Likewise, an unpaid invoice may still affect accrual-basis books even though no payment record exists.

Paper and electronic records

Source documents may be paper or electronic. Emails, PDFs, portal downloads, electronic receipts, images, and system-generated reports can be valid records when they remain accurate, readable, accessible, and organized.

Good practices include:

• Use consistent file names and folders by year and transaction type.
• Attach documents to the accounting transaction when practical.
• Preserve the original file rather than only a cropped screenshot.
• Back up records outside a single device or employee mailbox.
• Restrict deletion and document material changes.
• Export platform and processor reports before access expires.
• Protect documents containing bank, payroll, customer, or taxpayer information.

Tax and California recordkeeping

The IRS explains that purchases, sales, payroll, and other business transactions generate supporting documents, including invoices, receipts, deposit slips, and canceled checks. These records support entries in the books and amounts reported on tax returns. The IRS generally allows any recordkeeping system that clearly shows income and expenses.

The California FTB also requires taxpayers to retain records supporting income, deductions, adjustments, and credits. Its usual examination period may be four years, and certain records should be kept longer. Property records may be needed until the asset is disposed of and the period for the related return has expired. Payroll, entity, carryover, and unresolved audit records can have separate retention needs.

Common source-document mistakes

• Treating the bank description as a complete receipt.
• Keeping only a payment confirmation without item detail.
• Failing to document business purpose for meals, travel, gifts, or vehicle use.
• Using an estimate when an invoice or receipt could be obtained.
• Entering both a bill and its payment as expenses.
• Altering an invoice without a credit memo or revision history.
• Mixing personal and business purchases without identifying the personal part.
• Throwing away asset records after the current-year return is filed.

Frequently asked questions

Related terms

• Audit Trail
• Transaction Categorization
• Journal Entry
• Bank Reconciliation
• Accounts Payable
• Gross Revenue

Official sources

• IRS — What kind of records should I keep?
• IRS — Publication 583, Starting a Business and Keeping Records
• IRS — Topic No. 305, Recordkeeping
• IRS — How long should I keep records?
• California FTB — Keeping your tax records

The definitions and examples on this page are for informational purposes only and do not constitute tax advice. Tax laws change frequently and individual circumstances vary. Consult a qualified tax professional before making decisions based on this content.