The source document used to make entries in the purchase day book is

The source document used to make entries in the purchase day book is an invoice. An invoice is a document issued by a supplier of goods or services to a buyer, indicating the products, quantities, and agreed prices for products or services that the supplier has already provided to the buyer. When a business receives an invoice from a supplier for goods purchased on credit, the details of the invoice are recorded in the purchase day book, which is a book used to record all purchases made on credit by the business. This record is important for accurate record-keeping, tracking purchases, and monitoring the company's financial performance.