وَعَن سهل بنِ سعد: أَنَّ رَجُلًا اطَّلَعَ فِي جُحْرٍ فِي بَابِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَمَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مِدْرًى يَحُكُّ بِهِ رَأْسَهُ فَقَالَ: «لَوْ أَعْلَمُ أَنَّكَ تنظُرُني لطَعَنْتُ بهِ فِي عَيْنَيْكَ إِنَّمَا جُعِلَ الِاسْتِئْذَانُ مِنْ أَجْلِ الْبَصَرِ»

Sahl b. Sa'd said that a man looked through a hole in God’s Messenger’s door when God’s Messenger had a spike with which he was scratching his head, so he said, “If I knew that you were seeing me I would poke it in your eyes, for asking permission has been appointed only on account of what people may see.” (Bukhari and Muslim.)