Musnad Ahmad 27:1346

Muhammad ibn Ubayd narrated to us, Ubayd Allah narrated to us, from Nafi, from Ibn Umar, that Umar gave a horse to be ridden in the way of Allah, and the Messenger of Allah (peace and blessings of Allah be upon him) gave it to a man. Umar came to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "Shall I buy the horse that I gave to be ridden?" He said: "Do not buy it, and do not go back on your charity."