Sahih Bukhari Volume 4, Book 52, Hadith Number 246.

Narated By ‘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 Apostle who said, “Don’t buy it and don’t take back your gift of charity.”