Abu Dawud Book 8, Hadith Number 2724.

Narated By Umayr, client of AbulLahm : I was present at Khaybar along with my masters who spoke about me to the Apostle of Allah (PBUH). He ordered about me, and a sword was girded on me and I was trailing it. He was then informed that I was a slave. He, therefore, ordered that I should be given some inferior goods.