Sahih Muslim Book 012, Hadith Number 3949.
Chapter : Disapproval of buying the donated property by the donor himself.
Umar b. Khattab (Allah be pleased with him) reported: I donated a pedigree horse in the path of Allah. Its possessor made it languish. I thought that he would sell it at a cheap price. I asked Allah's Messenger (may peace be upon him) about it, whereupon he said: Don't buy it and do not get back your charity, for one who gets back the charity is like a dog who swallows its vomit. This hadith has been narrated on the authority of Malik b. Anas with the same chain of transmitters but with this addition: "Don't buy that even if he gives you for one dirham."