Sahih Bukhari Volume 7, Book 77, Hadith 5926

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 ‏"

Narrated By 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."