The Prophet (ﷺ) said: No sadaqah is due on a horse or a slave except that given at the breaking of the fast (at the end of Ramadan).
Sunan Abi Dawud Book 9, Hadith 39
← Previous Hadith ‘Umar bin Al Khattab gave a horse as alms in the way of Allah. He then found it being sold, and intended to buy it. So he asked the Messenger of Allah (ﷺ) about this. He said Do not buy it, and do not take back your sadaqah.

Sunan Abi Dawud Book 9, Hadith 38 Next Hadith → The Messenger of Allah(ﷺ) as saying No sadaqah is due from a Muslim on his slave or his horse.

Sunan Abi Dawud Book 9, Hadith 40