Sahih Muslim Book 012, Hadith Number 3950.
Chapter : Disapproval of buying the donated property by the donor himself.
Zaid b. Aslam reported on the authority of his father that 'Umar (Allah be pleased with him) donated a horse in the path of Allah. He found that it had languished in the hand of its possessor, and he was a man of meagre resources He (Hadrat 'Umar) intended to buy it. He came to Allah's Messenger (may peace be upon him) and made a mention of that to him, whereupon he said: Don't buy that even if you get it for a dirham for he who gets back the charity is like a dog which swallows its vomit.