Quran 4 : 86
بسم الله الرحمن الرحيم

وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّوا بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَاۗ إِنَّ اللَّهَ كَانَ عَلَىٰ كُلِّ شَيْءٍ حَسِيبًا

AUR JAB SALAM KIYA JAE TUM PAR KISI LAFZ SE TO TUM JAWAB DO USSE BEHTAR YA USIKO DOHRA DO BESHAK ALLAH HAR CHIZ KA HISAB LENEWALA HAY.

When you are greeted with a salutation then return it with a better one, or at least the same. Surely Allah takes good count of everything.