Asim bin Damrah (may Allah have mercy on him) narrates from Sayyiduna Ali (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "I have exempted horses and slaves from zakah. So pay the zakah of silver, one dirham out of every forty dirhams. There is no zakah on one hundred and ninety dirhams until they become a full two hundred; when they reach two hundred, five dirhams of zakah are due on them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7520