Surah An-Nisa, Verse 86:
وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّوا بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا إِنَّ اللَّهَ كَانَ عَلَىٰ كُلِّ شَيْءٍ حَسِيبًا

When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally. Certainly, Allah is Ever a Careful Account Taker of all things.

(English – Mohsin Khan)