Sunan an-Nasa'i 4859

It was narrated from Sahl bin Sa'd As-Saidi that:a man looked through a hole in the door of the Prophet, who had with him a kind of comb with which he was scratching his head, When the Messenger of Allah saw him he said: "If I had known that you were watching me, I would have stabbed you in the eye with this. This rule of asking permission has been ordained so that one may not look unlawfully (into people's houses)