وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّواْ بِأَحۡسَنَ مِنۡهَآ أَوۡ رُدُّوهَآ ۗ إِنَّ ٱللَّهَ كَانَ عَلَىٰ كُلِّ شَىۡءٍ حَسِيبًا
When a (courteous) greeting is offered you, meet it with a greeting still more courteous, or (at least) of equal courtesy. Allah takes careful account of all things.<br/>
And when you are greeted with a greeting, as when it is said to you, 'Peace be upon you', greet, the one that greeted you, with better than it, by responding to him with, 'Peace be upon you, and God's mercy and blessings', or return it, by saying back to him what he said; in other words, it is a duty to greet in one of these two ways, the former being the preferred one. Surely God keeps count of, He holds [you] accountable for, all things, and requites accordingly, including things such as returning a greeting. The Sunna specifies that one should not return the greeting of an disbeliever, an innovator, a wicked person, and of the one that greets a person who is in the act of relieving himself, or one in the bath, or one eating - indeed it is actually disapproved with the exception of the last. To the disbeliever [who says 'peace be upon you'] one should simply say, 'And upon you'.