Yahya related to me that Malik said, "A man does not have to payzakat for the slaves of his slaves, or for some one employed by him,or for his wife's slaves, except for anyone who serves him and whoseservices are indispensable to him, in which case he must pay zakat. Hedoes not have to pay zakat for any of his slaves that are kafir andhave not become muslim, whether they be for trade or otherwise."
USC-MSA web (English) reference: Book 17, Hadith 58