Musannaf Ibn Abi Shayba 3:2433

Ibn Fudayl narrated to us, from al-A’mash, from Ibrahim, from ‘Alqama, from ‘Abdullah, who said: We used to greet the Prophet, peace and blessings of Allah be upon him, while he was in prayer before we went out to the Negus, and he would return the greeting to us. When we returned from the Negus, I greeted him, but he did not return it. He said: "Indeed, in prayer there is an occupation."