Shulchan Arukh, Orach Chayim 437/שולחן ערוך·אורח חיים תל״ז

1One Who Rents to His Fellow, Upon Whom Does the Obligation to Destroy Chametz Fall. Containg 4 paragraphs: When a person rents a house to a tenant before the onset of the night of the 14th of Nissan, and the tenant acquired [the rights to live in the house] through one of the processes through which the rental of a home is validated, if before he hands him the key, the responsibility falls on the landlord to inspect, and if the key is handed over to him, the responsibility falls on the tenant to inspect.

2If the tenant rents a house from his friend on the 14th of Nissan, and he does not know the landlord checked (for Chametz), if the landlord is in town, he asks if it has been inspected, and if he is not in town, his assumption is that it has been inspected, and he cancels it in his heart, and it is sufficient.

3The landlord rents a house to his friend with the presumption of it being inspected, and if it is found to be uninspected, it is the responsibility of the tenant to inspect, and the transaction cannot be retracted. This applies even in a place where it is customary to hire someone else to check, as it is a mitzvah to fulfill. Rema: Some say that the landlord must return the inspection fee to the tenant since it was explicitly stipulated that it would be inspected (HaMagid, Perek 2, and see above Siman 14).

4A house that was presumed to be inspected without the landlord checking, and they (a woman, a slave, or a minor) said, 'We inspected it,' they are believed. However, the minor must be of an age capable of inspection.