Musnad Ahmad 5:350

Ibn Numayr narrated to us, al-A‘mash narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, from the Prophet, peace and blessings of Allah be upon him, who said: "I have pardoned for you [the Zakat on] horses and slaves. And there is no Zakat on what is less than two hundred [dirhams]."