What was the advice given by the Prophet Muhammad (saw) concerning the horse `Umar had given in charity?
 Narrated Ibn `Umar:                     Once `Umar gave a horse in charity to be used in holy fighting. It had been given to him by Allah's Apostle . `Umar gave it to another man to ride. Then `Umar was informed that the man put the horse for sale, so he asked Allah's Apostle whether he could buy it. Allah's Apostle replied, "You should not buy it, for you should not take back what you have given in charity."