"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." [Soorah an-Nisaa 04:86]
They reported from Abu Hurayrah (may Allaah be pleased with him) that a man passed by the Messenger of Allaah ﷺ whilst he was sitting with some others, and said: "As Salaam ‘alaykum (peace be upon you)." The Prophet ﷺ said, "[He will have] ten hasanaat (rewards)."
Another man passed by and said "As Salaam ‘alaykum wa rahmatullaah (peace be upon you and the mercy of Allaah)." The Prophet ﷺ said, "[He will have] twenty hasanaat."