Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "There is no zakah on horses and slaves, except that on slaves there is Sadaqat al-Fitr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7401