Sahih al-Bukhari 6901
Narrated Sahl bin Sa'd As-Sa'idi:
A man peeped through a hole in the door of Allah's Messenger (ﷺ)'s house, 
     and at that time, Allah's Messenger (ﷺ) had a Midri (an iron comb or bar) 
     with which he was rubbing his head. So when Allah's Messenger (ﷺ) saw him, 
     he said (to him), "If I had been sure that you were looking at me 
     (through the door), I would have poked your eye with this (sharp iron 
     bar)." Allah's Messenger (ﷺ) added, "The asking for permission to enter has 
     been enjoined so that one may not look unlawfully (at what there is in
     the house without the permission of its people)."
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، عَنِ ابْنِ شِهَابٍ، أَنَّ سَهْلَ بْنَ سَعْدٍ السَّاعِدِيَّ، أَخْبَرَهُ أَنَّ رَجُلاً اطَّلَعَ فِي جُحْرٍ فِي باب رَسُولِ اللَّهِ صلى الله عليه وسلم وَمَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم مِدْرًى يَحُكُّ بِهِ رَأْسَهُ، فَلَمَّا رَآهُ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ لَوْ أَعْلَمُ أَنْ تَنْتَظِرَنِي لَطَعَنْتُ بِهِ فِي عَيْنَيْكَ ‏"‏‏.‏ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّمَا جُعِلَ الإِذْنُ مِنْ قِبَلِ الْبَصَرِ ‏"‏‏.‏