Musnad Abu Ya'la al-Mawsili 152:120

Abu Khaythama narrated to us, Muhammad ibn Fudayl narrated to us, al-A'mash narrated to us, from Ibrahim, from 'Alqama, from 'Abdullah, who said: We used to greet the Messenger of Allah (may Allah bless him and grant him peace) 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. So we said: "O Messenger of Allah, we used to greet you in prayer and you would return it to us." He said: "Indeed, in prayer there is an occupation."