Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 215
Narrated 'Abdullah bin 'Umar:
'Umar gave a horse to be used in Allah's Cause, but later on he found it being sold. So, he intended to buy it and asked Allah's Apostle who said, "Don't buy it and don't take back your gift of charity."