Abu Dawud Book 2, Hadith Number 714.

Narated By Abdullah ibn Abbas : I came riding a donkey. Another version has: Ibn Abbas said: When I was near the age of the puberty I came riding a she-ass and found the Apostle of Allah (PBUH) leading the people in prayer at Mina. I passed in front of a part of the row (of worshippers), and dismounting left my she-ass for grazing in the pasture, and I joined the row, and no one objected to that.