AN NISAA' — Ayat 86

وَإِذَا حُيِّيْتُم بِتَحِيَّةٍ فَحَيُّواْ بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا إِنَّ اللّهَ كَانَ عَلَى كُلِّ شَيْءٍ حَسِيبًا

And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner]. Indeed, Allah is ever, over all things, an Accountant.