When underwriting a loan, large or unusual deposits that are not readily identifiable (such as payroll deposits) must be sourced and documented. This is to ensure the funds are not borrowed or from an unacceptable source.

“If a bank statement reveals a large deposit that is not payroll-related, the lender must document the source of funds.”

— Fannie Mae Selling Guide, B3-4.2-02: Depository Accounts