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."  

أَنَّ رَجُلاً اطَّلَعَ مِنْ جُحْرٍ فِي دَارِ النَّبِيِّ ﷺ وَالنَّبِيُّ ﷺ يَحُكُّ رَأْسَهُ بِالْمِدْرَى فَقَالَ لَوْ عَلِمْتُ أَنَّكَ تَنْظُرُ لَطَعَنْتُ بِهَا فِي عَيْنِكَ إِنَّمَا جُعِلَ الإِذْنُ مِنْ قِبَلِ الأَبْصَارِ