Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 820
Narrated Ibn 'Abbas:
Once I came riding a she-ass and I, then, had just attained the age of puberty. Allah's Apostle was leading the people in prayer at Mina facing no wall. I passed in front of the row and let loose the she-ass for grazing and joined the row and no one objected to my deed.