Sahih Muslim Book 004, Hadith Number 1096.
Chapter : Sitting on the buttocks.
Abdullah (b. Masu'd) reported: We used to greet the Messenger of Allah (may peace be upon him) 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 Allah, we used to greet you when you were engaged in prayer and you would respond to us. He replied: Prayer demands whole attention.