Musnad Ahmad 2:85

Sufyan narrated to us, from Zayd ibn Aslam, from his father: That Umar gave a horse to be ridden in the way of Allah, the Almighty and Majestic. Then he saw it, or some of its offspring, being sold, and he wanted to buy it. So he asked the Prophet, peace and blessings of Allah be upon him, about it. He said: "Leave it, and it will come to you, or you will meet it altogether." And he said another time: He forbade him, and said: "Do not buy it, and do not return to your charity."