Musnad Ahmad 2:177

Waki’ narrated to us, Hisham bin Sa’d narrated to us, from Zayd bin Aslam, from his father, from ‘Umar: That he found a horse that he had given for the sake of Allah being sold in the market. He wanted to buy it, so he asked the Prophet, peace and blessings of Allah be upon him. He forbade him and said: "Do not return to your charity."