The Prophet ﷺ said, "There is no zakat due upon a Muslim for his horse or his slave."
حدثنا يحيى قال حدثنا خثيم بن عراك قال حدثني أبي
عن النبي ﷺ ليس على المسلم في فرسه ولا مملوكه صدقة
Its isnad is sahih, mentioned previously in 2/407-410-420.