Surah An-Nur (The Light), Verse 27

یٰۤاَیُّهَا الَّذِیْنَ اٰمَنُوْا لَا تَدْخُلُوْا بُیُوْتًا غَیْرَ بُیُوْتِكُمْ حَتّٰی تَسْتَاْنِسُوْا وَتُسَلِّمُوْا عَلٰۤی اَهْلِهَا ذٰلِكُمْ خَیْرٌ لَّكُمْ لَعَلَّكُمْ تَذَكَّرُوْنَ ۟

Yaaa aiyuhal lazeena aamanoo laa tadkhuloo buyootan ghaira buyootikum hatta tastaanisoo wa tusallimoo 'allaa ahlihaa; zaalikum khairul lakum la'allakum tazakkaroon

“O ye who believe! enter not houses other than your own, until ye have asked permission and saluted those in them: that is best for you, in order that ye may heed (what is seemly).”

Quran 24:27 (Yusuf Ali Translation)