A voucher is documentary evidence in support of a transaction in the books of account. The act of establishing the accuracy and authenticity of an entry in the original books of account is called vouching. Vouching means not only to substantiate an entry in the books of account with any book, contracts, but also to see that the transaction had been properly authorized, recorded and entered in the books of accounts, e.g., verification of entries in the invoices book with the invoices, checking of the cash receipts with the counterfoils of the receipt book, the checking of the cash payments with the receipts issued by the payee and so on.
Vouching means testing the truth of the entry appearing in the books of original entry. The success of an auditor in vouching depends upon his intelligence, critical bent of mind, common sense, observation and tact with which he handles his work. He should go beyond the books of account and go to the source of a transaction.