Sahih Muslim Book 4, Hadith Number 1020.

Chapter : The excellence of the prayer and the way it is begun and the excellence of Ruku’ and moderation in it and prostration and moderation in it, etc.

Abdullah b. Abbas reported that he came riding on a donkey, and the Messenger of Allah (may peace be upon him) was leading the people in prayer at Mina on the occasion of the Farewell Pilgrimage and (the narrator) reported: The donkey passed in front of the row and then he got down from it And joined the row along with the people.