Sahih al-Bukhari 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."
حَدَّثَنَا آدَمُ بْنُ أَبِي إِيَاسٍ، حَدَّثَنَا ابْنُ أَبِي ذِئْبٍ، عَنِ الزُّهْرِيِّ، عَنْ سَهْلِ بْنِ سَعْدٍ، أَنَّ رَجُلاً، اطَّلَعَ مِنْ جُحْرٍ فِي دَارِ النَّبِيِّ صلى الله عليه وسلم وَالنَّبِيُّ صلى الله عليه وسلم يَحُكُّ رَأْسَهُ بِالْمِدْرَى فَقَالَ ‏
"‏ لَوْ عَلِمْتُ أَنَّكَ تَنْظُرُ لَطَعَنْتُ بِهَا فِي عَيْنِكَ، إِنَّمَا جُعِلَ الإِذْنُ مِنْ قِبَلِ الأَبْصَارِ ‏"‏‏.‏