From Sahih Bukhari (Hadith 3002):
Narrated `Abdullah bin `Umar:`Umar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Allah's Messenger (ﷺ) who said, "Don't buy it and don't take back your gift of charity
Reference: Bukhari 3002