Umar b. Khattab (Allah be pleased with him) reported: I donated a pedigree horse in the path of Allah. Its possesser made it languish. I thought that he would sell it at a cheap price. I asked Allah's Menengsr (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."
Sahih Muslim, Book 012, Number 3949