Musnad Ahmad 26:16

Muhammad ibn Fudayl narrated to us, al-A’mash narrated to us, from Ibrahim, from ‘Alqamah, from ‘Abdullah, who said: We used to greet the Messenger of Allah (peace and blessings of Allah be upon him) while he was in prayer, and he would return the greeting to us. When we returned from the presence of the Negus, we greeted him, but he did not return the greeting to us. We said: "O Messenger of Allah, we used to greet you in the prayer and you would return it to us?" He said: "Indeed, in the prayer, there is an occupation."