‘Abdallah b. Mas'ud said:
We used to greet the Prophet 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 God, we used to greet you when you were engaged in prayer and you would respond to us.” He replied, “Prayer demands one’s whole attention.”
* The reference is to the return of those who had gone from Mecca to Abyssinia when Muslims were being persecuted.
(Bukhari and Muslim.)