Surah No. 24, An Noor, Ayat No. 27

يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تَدْخُلُوا بُيُوتًا غَيْرَ بُيُوتِكُمْ حَتَّى تَسْتَأْنِسُوا وَتُسَلِّمُوا عَلَى

o أَهْلِهَا ذَلِكُمْ خَيْرٌ لَّكُمْ لَعَلَّكُمْ تَذَكَّرُونَ

Translation :

O you who believe! Do not Enter other's houses than your own, until you have asked for permission and greeted (with Salam) to those in them : that is best for you in order that you may remember.