From Sahih Bukhari (Hadith 76):
Narrated Ibn `Abbas:Once I came riding a she-ass and had (just) attained the age of puberty. Allah's Messenger (ﷺ) 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
Reference: Bukhari 76