What did the Prophet (saw) advise when `Umar bin Al-Khattab saw the horse he gave to be used in Allah's cause being sold?
 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."