The Holy Quran, Surah Al Nur, 24:27-28
"O you who believe! enter not houses other than your own, until you have asked permission and saluted those in them: that is best for you, in order that you may heed (what is seemly). If you find no one in the house, enter not until permission is given to you: if you are asked to go back, go back: that makes for greater purity for yourselves: and Allah knows well all that ye do."
