Musnad Abu Ya'la al-Mawsili 154:211

On the authority of Abdullah, that Umar ibn al-Khattab gave a horse in the way of Allah. The Messenger of Allah (peace be upon him) mounted a man upon that horse. Umar found him selling it, so Umar said to the Messenger of Allah (peace be upon him), "The one whom you mounted upon the horse, I found him selling it. Shall I buy it, O Messenger of Allah?" He said, "Do not buy it, and do not return to your charity."