It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to order us that, regarding the male and female slaves of a man or woman who are his personal (long-held) property, are engaged in his work, and whom he does not intend to sell, we should not pay any zakah on them, and he used to order us to pay zakah from that (property) which is kept ready for sale.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6999