Musnad Abu Ya'la al-Mawsili 155:299

Al-Abbas ibn al-Walid narrated to us, Wahib narrated to us, from Ubayd Allah ibn Umar, from a man, from Makhul, from Arak ibn Malik, from Abu Hurayrah, who said: "There is no Zakat (obligatory charity) on horses and slaves, except for the Zakat al-Fitr."