Book 9: Virtues of the Prayer Hall (Sutra of the Musalla)
Hadith 472 (Volume 1)
Narrated Ibn 'Abbas:
Once I came riding a she-ass when I had just attained the age of puberty. Allah's Apostle was offering the prayer at Mina with no wall in front of him and I passed in front of some of the row. There I dismounted and let my she-ass loose to graze and entered the row and nobody objected to me about it.