Sahih Bukhari Volume 3, Book 29, Hadith Number 81.

Narated By ‘Abdullah bin ‘Abbas : I came riding on my she-ass and had (just) then attained the age of puberty. Allah’s Apostle was praying at Mina. I passed in front of a part of the first row and then dismounted from it, and the animal started grazing. I aligned with the people behind Allah’s Apostle (The sub-narrator added that happened in Mina during the Prophet’s Hajjat-ul-wada.)