Abdullah bin Umar (may Allah be pleased with them both) said: There is no zakah on the wealth of a slave or of a mukatab (a slave contracted to buy his freedom).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7351
Sayyiduna Jabir (may Allah be pleased with him) said: There is no zakah on the wealth of a mukatab or a slave until they are set free.
This has also been narrated about the mukatab as marfu' (from the Prophet) through Abdullah bin Bazi' from Ibn Jurayj, but it is weak; the correct version is mawquf (the Companion's own statement).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7352