Sahih Bukhari
Dress
Volume: 007 / Book Number: 072 / Hadith Number: 807
Narrated 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."