Sahih Muslim Book 12, Hadith Number 3952.

Chapter : Disapproval of buying the donated property by the donor himself.

Ibn ‘Umar reported that ‘Umar b. al-Khattib (Allah be pleased with him) donated a horse in the path of Allah and (later on) he found it being sold, and he decided to buy that. He asked the Messenger of Allah (may peace be upon him) about it, whereupon he (the Holy prophet) said: Don’t buy that and do not get back what you gave in charity.