It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once Sayyiduna Umar (may Allah be pleased with him) gave a horse to someone in the way of Allah for riding. The Prophet (peace and blessings of Allah be upon him) gave that horse to a man. Then Sayyiduna Umar (may Allah be pleased with him) came to the presence of the Messenger (peace and blessings of Allah be upon him) and submitted, "The horse which I gave for riding, can I buy it back?" The Prophet (peace and blessings of Allah be upon him) said, "Do not buy it, and do not take back your charity."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5796