Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gave a man a horse as charity in the path of Allah; then Umar (may Allah be pleased with him) saw him selling it. He came to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, I gave so-and-so a horse as charity, and I have seen him selling it; shall I buy it? The Messenger of Allah (peace and blessings be upon him) said: "Do not buy it, and do not go back on your charity."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13188