The responsibility to prove entries, deductions, and statements made on your tax returns is known as the burden of proof. You must be able to prove (substantiate) certain elements of expenses to deduct them.
Generally, taxpayers meet their burden of proof by having the information and receipts (where needed) for the expenses. You should keep adequate records to prove your expenses or have sufficient evidence that will support your own statement. You generally must have documentary evidence, such as receipts, canceled checks, or bills, to support your expenses. Additional evidence is required for travel, entertainment, gifts, and auto expenses.