Sahih Al Bukhari Hadith Number 861

Narrated Ibn `Abbas: Once I came riding a she-ass and I, then, had just attained the age of puberty. Allah’s Messenger (peace be upon him) 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.

10  Call to Prayers (Adhaan)
Hadith number 861
Volume no 1
Chapter: 161 (The ablution for boys (youngsters). When they should perform Ghusl (take a bath) and Tuhur (purification). Their attendance at congregational prayers and their rows in the prayers.)