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