Sahih Bukhari | Volume 7 | Hadith Number 807

Narrated by Sa'd
A man peeped into the house of the Prophet through a hole while the Prophet was scratching his head with a Midrai (a certain kind of comb). On that the Prophet said (to him), "If I had known you had been looking, then I would have pierced your eye with that instrument, because the asking of permission has been ordained so that one would not see things unlawfully."