Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 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."