Is a praying person who does not return the greeting sinful—knowing that returning the greeting is a communal obligation (fard kifayah)—or is the one who extended the greeting to those praying sinful?

It is permissible to greet a person who is praying, and the praying person should return the greeting by gesturing. If he returns it verbally, his prayer becomes invalid. It is recommended for the praying person to return the greeting verbally after finishing the prayer. Some scholars are of the opinion that there is no sin upon the praying person if he does not return the greeting during or after the prayer.