رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 871
The Book of Greetings
Seeking Permission to Enter (Someone's House) and Manners Relating to It
وَعَنْ سَهْلِ بْنِ سَعْدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : إِنَّمَا جُعِلَ الاِسْتِئْذَانُ مِنْ أَجْلِ الْبَصَرِ ((مُتَّفَقٌ عَلَيْهِ)).
Sahl bin Sa'd (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "Seeking permission to enter (somebody's house) has been prescribed in order to restrain the eyes (from looking at something we are not supposed to look at)." [Al-Bukhari and Muslim].
Riyad as-Salihin, 871