How did Prophet Muhammad (peace be upon him) advise `Umar bin Al-Khattab when he wanted to purchase the horse he had previously given in Allah's Cause?
 Narrated `Abdullah bin `Umar:                     `Umar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Allah's Apostle who said, "Don't buy it and don't take back your gift of charity."