From Sahih Bukhari (Hadith 861):
Narrated Ibn `Abbas:Once I came riding a she-ass and I, then, had just attained the age of puberty. Allah's Messenger (ﷺ) was leading the people in prayer at Mina facing no wall. I passed in front of the row and let loose the sheass for grazing and joined the row and no one objected to my deed
Reference: Bukhari 861