Abu Dawud Book 8, Hadith Number 2566.

Narated By Buraydah ibn al-Hasib : While the Apostle of Allah (PBUH) was walking a man who had an ass came to him and said: Apostle of Allah, ride; and the man moved to the back of the animal. The Apostle of Allah (PBUH) said: No, you have more right to ride in front on your animal than me unless you grant that right to me. He said: I grant it to you. So he mounted.