What document serves as initial evidence of a business transaction?

The initial evidence of a business transaction is captured by a source document. Source documents are the original records that provide proof of the transaction. They can include invoices, receipts, purchase orders, and bank statements, among others. These documents serve as the foundation for recording transactions in the accounting system, as they contain essential details such as dates, amounts, and the parties involved in the transaction.

After the source documents are established, transactions are recorded in journals, which are the books of original entry. From the journals, information is then posted to ledgers, which categorize the transactions and provide balances for accounts. The trial balance is generated from the ledgers at the end of an accounting period to ensure that debits equal credits. However, none of these subsequent documents serve as the initial proof of the transaction itself, which is why a source document is the correct answer to the question.