Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 214
Narrated 'Umar bin Al-Khattab:
I gave a horse to be used in Allah's Cause, but later on I saw it being sold. I asked the Prophet whether I could buy it. He said, "Don't buy it and don't take back your gift of charity."