Sahih al-Bukhari 2623

Yahya ibn Qazaʿa narrated to us: Malik narrated to us, from Zayd ibn Aslam, from his father, who said: I heard ʿUmar ibn al-Khattab, may Allah be pleased with him, say: I gave a horse in the cause of Allah. The person who had it failed to care for it, so I wanted to buy it from him, thinking he would sell it cheaply. I asked the Prophet ﷺ about that, and he said: “Do not buy it, even if he offers it to you for a single dirham, for one who takes back his charity is like a dog returning to its vomit.”