In the
Help Center is just says 'Proof of the expense should be attached to the form. Original documents are preferred, if possible. These could include sales receipts, bills, or invoices.'.
In practice, we've generally obtained a copy of the lease and have never had problems in the audit on that point. If you can't get an actual lease or rental agreement then you should try for a receipt or letter from the landlord indicating the rent amount and that the FO recipient is responsible for it.