Asim bin Damrah (may Allah have mercy on him) said that Sayyiduna Ali (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "I have exempted you from the zakah of slaves and horses. Pay one dirham as zakah out of every forty dirhams. There is no zakah on one hundred and ninety, and when they reach two hundred dirhams, the zakah is five dirhams."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7406