It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not enter the houses of the Ahl al-Dhimmah (non-Muslim protected citizens) without permission."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5829