An-Nisa 4:86

Wa-itha huyyeetum bitahiyyatinfahayyoo bi-ahsana minha aw ruddoohainna Allaha kana AAala kulli shay-in haseeba

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 Allāh is ever, over all things, an Accountant.

When you are greeted with a salutation, greet with one better than it, or return the same. Surely, Allah is the One who takes account of everything.