Musnad Ahmad 25:820

Hassan narrated to us, Hammad ibn Salamah narrated to us, from Ammar ibn Abi Ammar, that Ibn Abbas said: I was with my father at the Messenger of Allah (peace and blessings of Allah be upon him), and there was a man with him speaking to him privately, and he was as if he were turning away from my father. We left from his presence, and my father said to me: "O my son, did you not see your cousin as if he were turning away from me?" I said: "O my father, there was a man with him speaking to him privately." He said: So we returned to the Prophet (peace and blessings of Allah be upon him), and my father said: "O Messenger of Allah, I said to Abdullah such and such, and he informed me that there was a man with you speaking to you privately. Was there anyone with you?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "And did you see him, O Abdullah?" I said: "Yes." He said: "That was Gabriel, and he is the one who occupied me from you."