Musnad Abu Ya'la al-Mawsili 2:27

Narrated to us Ibn Numayr, narrated to us my father, narrated to us Hisham bin Sa'd, from Zayd bin Aslam, from his father, from Umar: That he found a horse which he had previously carried [a rider] upon in the way of Allah, and he wanted to purchase it. So he asked the Messenger of Allah, peace and blessings of Allah be upon him, and he forbade him from it.