Zayd bin Aslam (may Allah have mercy on him) said: I heard my father say that Umar bin al-Khattab (may Allah be pleased with him) said: I gave a horse (to be ridden) in the way of Allah, then I saw it being sold, so I asked the Messenger of Allah (peace and blessings be upon him): Shall I buy it? He (peace and blessings be upon him) said: "Do not buy it, and do not go back on your charity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7631