What is written evidence?

Records must be kept for business mileage or expenses. Receipts, invoices, bank statements, etc. can be used as proof of actual expenses. If you're claiming standard mileage, you'll need some kind of log like a ledger, spreadsheet, or app that includes the date, place, total miles driven, and the purpose of the business conducted there.

See IRS Publication 463 Table 5-2 for an example.