Zayd bin Aslam narrates from his father that he said: I heard Umar bin al-Khattab (may Allah be pleased with him) say: I gave a fine horse in the way of Allah, and its owner (to whom it was given) neglected it. I wanted to buy it from him, and I thought that he would sell it to me cheaply, so I asked the Messenger of Allah (peace and blessings be upon him) about it, and he (peace and blessings be upon him) said: "Do not buy it from him even if he gives it to you for a single dirham, for the one who goes back on his charity is like the dog that goes back to its vomit."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7633