Hadith 1613

'Umar bin Al-Khattab (May Allah be pleased with him) said: I donated a horse in the way of Allah. Its new possessor did not treat it properly. I made my mind to buy it because I thought that he would sell it at a cheap price (now that it became weak and emaciated). I asked the Prophet (PBUH) about it, whereupon he said, "Do not buy it and do not get back your charity, for one who gets back the charity is like a dog that eats its own vomit."

[Al-Bukhari and Muslim]