(You Shall Be Courteous)
When greeted with a greeting, you shall respond with a better greeting or at least an equal one. God reckons all things.
وَإِذا حُيّيتُم بِتَحِيَّةٍ فَحَيّوا بِأَحسَنَ مِنها أَو رُدّوها إِنَّ اللَّهَ كانَ عَلىٰ كُلِّ شَىءٍ حَسيبًا
wa izaa huyyeetum bitahaiyyatin fahaiyoo bi ahsana minhaaa aw ruddoohaa; innal laaha kaana ‘alaa kulli shai’in Haseeba