Musnad Abu Ya'la al-Mawsili 155:298

Al-Abbas ibn al-Walid al-Narsi narrated to us, Wahib narrated to us, from Khuthaym ibn Arak ibn Malik, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "There is no Zakat (charity) due on a Muslim's slave, nor on his horse."