Musnad Ahmad 933:1

Abdullah narrated to us, Muhammad ibn Abd al-Rahim Abu Yahya al-Bazzaz narrated to us, Yunus ibn Muhammad narrated to us, Mu'adh ibn Muhammad ibn Mu'adh ibn Muhammad ibn Ubayy ibn Ka'b narrated to us, my father Muhammad ibn Mu'adh narrated to me, from Mu'adh, from Muhammad, from Ubayy ibn Ka'b: That Abu Hurayrah was bold in asking the Messenger of Allah (may Allah bless him and grant him peace) about things that no one else asked him about. He said: "O Messenger of Allah, what is the first thing you saw regarding the matter of Prophethood?" The Messenger of Allah (may Allah bless him and grant him peace) sat up straight and said: "You have asked, O Abu Hurayrah. I was in the desert, ten years and some months old, when there was speech above my head, and there was a man saying to another: 'Is it him?' He said: 'Yes.' They approached me with faces I had never seen on any creature before, and scents I had never found from any creature before, and garments I had never seen on anyone before. They came toward me walking, until each of them took hold of my upper arms; I did not feel the touch of their grip. One of them said to his companion: 'Lay him down.' So they laid me down without force or struggle. One of them said to his companion: 'Split his chest.' One of them leaned toward my chest and split it, as I saw, without blood or pain. He said to him: 'Remove the rancor and the envy.' He brought out something like a blood clot, then cast it away and threw it. He said to him: 'Insert kindness and mercy.' And there was something like what he had brought out, resembling silver. Then he shook the big toe of my right foot and said: 'Go forth and be safe.' So I returned with it, going forth with tenderness for the young and mercy for the old."