Sahih Muslim Book 4, Hadith Number 1019.

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.

Ibn ‘Abbas reported: I came riding on a she-ass, and I was on the threshold of maturity, and the Messenger of Allah (may peace be upon him) was leading people in prayer at Mina. I passed in front of the row and got down, and sent the she-ass for grazing and joined the row, and nobody made any objection to it.