From Sahih Bukhari (Hadith 5924):
Narrated Sa`d:A man peeped into the house of the Prophet (ﷺ) through a hole while the Prophet (ﷺ) was scratching his head with a Midrai (a certain kind of comb). On that the Prophet (ﷺ) 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
Reference: Bukhari 5924