(86) 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, Allah is ever, over all things, an Accountant.
The Messenger of Allah (PBUH) commanded us to do seven things: attend funerals, visit the sick, bless one who sneezes, accept invitations, support the oppressed, fulfill oaths, and return greetings.
A man asked the Prophet (PBUH): Which deed is best? He said: To feed the poor and greet those you know and those you do not know.