Musnad Ahmad 27:728

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar, that Umar, may Allah be pleased with him, gave a horse [to be used] for the sake of Allah. Umar gave it to the Messenger of Allah, peace and blessings of Allah be upon him, so that he might mount a man upon it. Then Umar was informed that the person had stopped it [from being used for jihad] to sell it. He said: So he asked the Prophet, peace and blessings of Allah be upon him, about buying it. He said: "Do not buy it, and do not return to your charity."