Sahih Muslim Book 25, Hadith Number 5367.
Chapter : It is forbidden to peep into the house of another person.
Sahl b. Sa’d as-Sa’idi reported that a person peeped through the hole of the door of Allah’s Messenger (may peace be upon him) and he had with him some pointed thing with which he had been adjusting (the hair of his head). Allah’s Messenger (may peace be upon him) said to him: If I were to know that you had been peeping. I would have thrust it in your eyes. Allah has prescribed seeking permission because of protection against glance.