Abdullah (b. Masu'd) reported: We used to greet the Messenger of Allaah (sallAllaahu alayhi wa sallam) while he was engaged in prayer and he would respond to our greeting. But when we returned from the Negus we greeted him and he did not respond to us; so we said: Messenger of Allaah. we used to greet you when you were engaged in prayer and you would respond to us. He replied: Prayer demands whole attention.