Musnad Ahmad 2:200

'Abd al-Rahman narrated to us, from Malik, from Zayd ibn Aslam, from his father, from 'Umar ibn al-Khattab, who said: I gave a horse to be ridden in the way of Allah, but its owner neglected it. I wanted to buy it, and I thought he would sell it cheaply. So I said: "I will ask the Messenger of Allah (peace and blessings of Allah be upon him) first." He said: "Do not buy it, even if he gives it to you for a dirham, for the one who takes back his charity is like a dog that returns to its vomit."