Sahih Bukhari Volume 002, Book 024, Hadith Number 567.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 24. Obligatory Charity Tax (Zakat)
Narated By 'Umar : Once I gave a horse in Allah's Cause (in charity) but that person did not take care of it. I intended to buy it, as I thought he would sell it at a low price. So, I asked the Prophet (p.b.u.h) about it. He said, "Neither buy, nor take back your alms which you have given, even if the seller were willing to sell it for one Dirham, for he who takes back his alms is like the one who swallows his own vomit."