Musnad Ahmad 1017:58

Zayd, who is Ibn al-Hubab, narrated to us, Husayn ibn Waqid narrated to me, Abdullah ibn Buraydah narrated to me, he said: I heard my father say: While the Messenger of Allah, peace and blessings of Allah be upon him, was walking, a man came with a donkey and said: "O Messenger of Allah, ride." The man then stepped back. The Messenger of Allah, peace and blessings of Allah be upon him, said: "No, you have more right to the front of your mount than I do, unless you grant it to me." He said: "I have indeed granted it to you." He said: So he rode it.