Chapter 25: The Book of Gifts
Salim reported from Ibn Umar (Allah be pleased with them) that 'Umar donated a horse in the path of Allah and then found it being sold, and he decided to buy that. He asked Allah' s Apostle (ﷺ) about it, whereupon Allah' s Messenger (ﷺ) said: Umar, do not get back what you gave as charity.
Sahih Muslim 1621c