source document
Before anything is written in the books, something usually happens in the real world that leaves a trace: a sales slip prints out, a supplier emails a bill, the bank sends a statement, a worker signs a timecard. A source document is that original piece of evidence — paper or electronic — proving a transaction took place. It is the seed from which an accounting entry grows.
Concretely, a source document records the who, what, when, and how much of an event. An invoice shows a sale and its amount; a receipt shows that cash changed hands; a deposit slip shows money put in the bank; a payroll register shows wages owed. The accountant reads the source document, decides which accounts it affects, and only then makes the journal entry. No source document, no reliable entry.
Source documents matter most when someone questions a number. Auditors, tax authorities, and managers all want to see the underlying evidence, not just the bookkeeper's word. This is why businesses keep documents for years. A frequent misconception: the entry in the books is not the proof — the source document is. The books summarize; the documents substantiate.
A café buys cups from a supplier. The supplier's invoice — dated, listing 500 cups at $0.30 each, total $150 — is the source document. The café files it, then records a $150 purchase and a $150 account payable based on it.
A supplier invoice acting as the source document for an entry.
The journal entry is a summary, not proof; the source document is the actual evidence auditors and tax authorities will ask to see.