Abdullah bin Nafi' narrates from a man who said: I asked Umar bin al-Khattab (may Allah be pleased with him): O Commander of the Believers! Is zakah due on a slave? He said: No. I said: Then on whom is the zakah due? He said: On his owner. Jabir (al-Hadhdha) said: I asked Ibn Umar (may Allah be pleased with them both): Is there zakah on a slave's wealth? He said: There is zakah on the wealth of every Muslim: five in every two hundred, and whatever exceeds that is by the same proportion.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7350