From Sahih Muslim (Hadith 1124):
Ibn 'Abbas reported:I came riding on a she-ass, and I was on the threshold of maturity, and the Messenger of Allah (ﷺ) 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
Reference: Muslim 1124