Zayd bin Aslam narrates from his father that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gave a horse in the way of Allah. Then he saw some of its offspring being sold, and he wanted to buy it, so he asked the Prophet (peace and blessings be upon him) about it, and 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 / 7632