Maliks Muwatta Book 17, Hadith Number 40.
Section : The Zakat on Slaves, Horses and Honey.
Yahya related to me from Malik that Abdullah ibn Abi Bakr ibn Amr ibn Hazim said, “A message came from Umar ibn Abd al-Aziz to my father when he was in Mina telling him not to take zakat from either honey or horses.”