Sahih Bukhari
Sahih Bukhari Volume 003, Book 047, Hadith Number 804.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 47. Gifts
Narated By '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."