Irak bin Malik (may Allah have mercy on him) said: I heard Abu Hurayrah (may Allah be pleased with him) narrate that he (peace and blessings be upon him) said: "There is no charity due on a slave except sadaqat al-fitr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7673