Musnad Abu Ya'la al-Mawsili 139:26

Ibrahim ibn Muhammad ibn ‘Ar‘arah narrated to us, Mu‘tamir narrated to us, from his father, who said: I heard Anas say: That the Prophet, peace and blessings of Allah be upon him, on the night he was taken on the Night Journey (Isra), passed by Musa while he was praying in his grave. Anas said: He mentioned that he was carried upon the Buraq, and he tethered the beast, or he said the horse. Abu Bakr said: "Describe it to me." The Messenger of Allah, peace and blessings of Allah be upon him, said—and he mentioned a word—and he said: "I bear witness that you are the Messenger of Allah." And Abu Bakr had already seen it.