Musnad Ahmad 26:334

'Abd al-Razzaq narrated to us, Sufyan informed us, from al-A'mash, from Ibrahim, who said: 'Abdullah said: We used to greet the Prophet, may Allah bless him and grant him peace, while in prayer, until we returned from the presence of the Negus. We greeted him, but he did not return the greeting to us, and he said: "Indeed, in prayer there is an occupation."