14 C.F.R. § 152.307

Each sponsor or planning agency shall retain, for a period of 3 years after the date of submission of the final expenditure report—

(a) Documentary evidence, such as invoices, cost estimates, and payrolls, supporting each item of project costs; and

(b) Evidence of all payments for items of project costs, including vouchers, cancelled checks or warrants, and receipts for cash payments.