Yahya related to me from Malik from Abdullah ibn Umar fromSulayman ibn Yasar from Irak ibn Malik from Abu Hurayra that theMessenger of Allah, may Allah bless him and grant him peace, said, "Amuslim does not have to pay any zakat on his slave or his horse."
USC-MSA web (English) reference: Book 17, Hadith 38
Arabic reference: Book 17, Hadith 614