Malik Muwatta: Hadith 650

Yahya related to me that Malik said, "A man does not have to pay zakat 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 whose services are indispensable to him, in which case he must pay zakat. He does not have to pay zakat for any of his slaves that are kafir and have not become muslim, whether they be for trade or otherwise."