The Messenger of Allah ﷺ said, "I have exempted horses and slaves from zakat. So pay the zakat on silver, one dirham for every forty dirhams. There is nothing due on one hundred and ninety, but when it reaches two hundred, then five dirhams are due on it."
قال رسول الله ﷺ: قد عفوت عن الخيل والرقيق، فهاتوا صدقة الرقة من كل أربعين درهما درهما، وليس في تسعين ومائة شيء فإذا بلغت مائتين ففيها خمسة دراهم.
Narrated by Abu Dawud (1574), Tirmidhi (620) from the hadith of Abu Awana, Nasai (2478), Ibn Majah (1790), and Imam Ahmad (711, 984), all of them through various routes from Abu Ishaq, from Asim bin Damra, with Harith appearing in some of the routes, both of them from Ali, who related it. Ibn Khuzayma (2284) graded it sahih. See its full referencing in the chapter on zakat on horses and slaves.