Musnad Abu Ya'la al-Mawsili 3:320

Abu Khaythama narrated to us, Sufyan ibn 'Uyayna narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "We have exempted you from the Zakat on horses and slaves."