Abu Dawud Book 36, Hadith Number 5104.

Narated By AbuUqbah : AbdurRahman ibn AbuUqbah quoted his father AbuUqbah who was a client from the people of Persia as saying: I was present at Uhud along with the Apostle of Allah (PBUH), and on smiting one of the polytheists I said: Take this from me who is the young Persian. The Apostle of Allah (PBUH) then turned to me and said: Why did you not say: Take this from me who is the young Ansari?