Sahih Bukhari
Gifts
Volume: 003 / Book Number: 047 / Hadith Number: 803
Narrated 'Umar bin Al-Khatab:
Once I gave a horse (for riding) in Allah's Cause. Later I saw it being sold. I asked Allah's Apostle (whether I could buy it). He said, "Don't buy it, for you should not get back what you have given in charity."