Sahih Bukhari Volume 8, Book 74, Hadith Number 258.

Narated By Sahl bin Sa’d : A man peeped through a round hole into the dwelling place of the Prophet, while the Prophet had a Midray (an iron comb) with which he was scratching his head. the Prophet said, ” Had known you were looking (through the hole), I would have pierced your eye with it (i.e., the comb).” Verily! The order of taking permission to enter has been enjoined because of that sight, (that one should not look unlawfully at the state of others). (See Hadith No. 807, Vol. 7)