Sahih al-Bukhari 3002
صحيح البخاري#3002SahihNarrated `Abdullah bin `Umar:`Umar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Allah's Messenger (ﷺ) who said, "Don't buy it and don't take back your gift of charity
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 56:211