What did Allah's Apostle (saw) advise `Umar to do when he found the horse he had given for Allah's Cause being sold?
 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."