Surah An-Nisa' (The Women), Verse 86

وَاِذَا حُیِّیْتُمْ بِتَحِیَّةٍ فَحَیُّوْا بِاَحْسَنَ مِنْهَاۤ اَوْ رُدُّوْهَا اِنَّ اللّٰهَ كَانَ عَلٰی كُلِّ شَیْءٍ حَسِیْبًا ۟

Wa izaa huyyeetum bitahaiyyatin fahaiyoo bi ahsana minhaaa aw ruddoohaa; innal laaha kaana 'alaa kulli shai'in Haseeba

“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.”

Quran 4:86 (Yusuf Ali Translation)