A source document is a paper or electronic document used to record a business transaction. The sales day book is a record of all credit sales made by a business over a period of time. Therefore, the source document for the sales day book is an invoice. An invoice is a document sent by a seller to a buyer that indicates the type, quantity, and price of goods or services sold, as well as any applicable taxes, discounts, and payment terms. It serves as proof of a sale and provides the necessary information to create an entry in the sales day book.