Musnad Ahmad 27:74

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Salim, from his father, that 'Umar ibn al-Khattab gave a horse to be ridden in the way of Allah. Then he found it being sold, so he asked the Prophet, peace and blessings of Allah be upon him, about buying it. The Prophet, peace and blessings of Allah be upon him, said: "Do not return to your charity."