(75) باب الاِمْتِشَاطِ
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: Sahih al-Bukhari 5924
In-book reference: Book 77, Hadith 139
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 72, Hadith 807