Musnad Ahmad 30:639

'Abd al-Razzaq narrated to us, Ma'mar and Ibn Jurayj narrated to us, from Isma'il ibn Umayyah, from Makhul, from 'Arak ibn Malik, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "There is no Zakat (charity) upon the believer for his slave nor for his horse."