Sahih Bukhari Volume 001, Book 003, Hadith Number 076.
Narated By Ibn 'Abbas : Once I came riding a she-ass and had (just) attained the age of puberty. Allah's Apostle was offering the prayer at Mina. There was no wall in front of him and I passed in front of some of the row while they were offering their prayers. There I let the she-ass loose to graze and entered the row, and nobody objected to it.