Musannaf Ibn Abi Shayba 28:524

Humaid ibn Abd al-Rahman narrated to us, from Hasan, from Abu Sa’id, from Muhammad ibn Ubayd Allah al-Thaqafi, who said: Umar had four thousand horses at al-Ara in Kufa, branded on their thighs for the sake of Allah. If a man’s stipend was his due, or if he was in need, he would give him a horse. Then he would say: "If you race it and exhaust it, or neglect it regarding fodder, then you are liable. But if you fight upon it and it is struck, or you are struck, then there is nothing upon you."