Maliks Muwatta Book 17, Hadith Number 38.
Section : The Zakat on Slaves, Horses and Honey.
Yahya related to me from Malik from Abdullah ibn Umar from Sulayman ibn Yasar from Irak ibn Malik from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “A muslim does not have to pay any zakat on his slave or his horse.”