Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 247

Narrated Aslam:

I heard 'Umar bin Al-Khattab saying, "I gave a horse to be ridden in Allah's Cause and the person who got it intended to sell it or neglected it. So, I wanted to buy it as I thought he would sell it cheap. I consulted the Prophet who said, "Do not buy it even if for one Dirham, because he who takes back his gift is like a dog swallowing its vomit."