وَإِذَا حُيِّيْتُم بِتَحِيَّةٍ فَحَيُّواْ بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا إِنَّ اللّهَ كَانَ عَلَى كُلِّ شَيْءٍ حَسِيبًا
[4:86]
And when you are greeted with a greeting, greet with a better (greeting) than it or return it; surely Allah takes account of all things.
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.
When ye are greeted with a greeting, greet ye with a better than it or return it. Lo! Allah taketh count of all things.