Sahih Muslim Book - 38, Hadith - 5639

وحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى ، أَخْبَرَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي يُونُسُ ، عَنِ ابْنِ شِهَابٍ ، أَنَّ سَهْلَ بْنَ سَعْدٍ الأَنْصَارِيَّ ، أَخْبَرَهُ ، أَنَّ رَجُلاً اطَّلَعَ مِنْ جُحْرٍ فِي بَابِ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، وَمَعَ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مِدْرًى يُرَجِّلُ بِهِ رَأْسَهُ ، فَقَالَ لَهُ رَسُولُ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : لَوْ أَعْلَمُ أَنَّكَ تَنْظُرُ ، طَعَنْتُ بِهِ فِي عَيْنِكَ ، إِنَّمَا جَعَلَ اللَّهُ الإِذْنَ مِنْ أَجْلِ الْبَصَرِ.

Sahl bin Sa'd As-Sa'idi Al-Ansari narrated that a man looked through a crack in the door of the Messenger of Allah (s.a.w), and the Messenger of Allah (s.a.w) had a comb with which he was combing his hair. The Messenger of Allah (s.a.w) said to him: "If I had known that you were looking at me I would have poked you in the eye with it. Allah has only enjoined seeking permission because of looking."