Musnad Abu Ya'la al-Mawsili 155:265

Abu Ma'mar narrated to us, Muhammad bin Fudayl narrated to us, from 'Umarah bin al-Qa'qa', from Abu Zur'ah, from Abu Hurayrah, who said: Gabriel sat with the Prophet, may Allah bless him and grant him peace, and he looked toward the sky. Suddenly, an angel was descending. Gabriel said to him, "This angel has not descended since he was created until this hour." When he descended, he said, "O Muhammad, your Lord has sent me to you: shall I make you a king, or a servant-messenger?" Gabriel said to him, "Be humble before your Lord, O Muhammad." He said, "Rather, a servant-messenger."