Ali bin Abi Talib (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "I have exempted the zakah of slaves and horses."
Imam al-Thawri (may Allah have mercy on him) said: He (peace and blessings be upon him) said: "So pay the zakah of your wealth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7407