Musnad Ahmad 5:702

Abdullah narrated to us, Muhammad ibn Ishkab narrated to me, Muhammad ibn Abi Ubaydah narrated to us, my father narrated to me, from al-A’mash, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have exempted horses and slaves from Zakat."