Musnad Ahmad 27:455

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Salim, from Ibn 'Umar, that 'Umar gave a horse of his to be used in the way of Allah. Then he saw it being sold, so he wanted to buy it. The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Do not return to your charity."