Musnad Abu Ya'la al-Mawsili 3:301

Al-Qawariri narrated to us, Muhammad ibn 'Abd al-Wahid ibn Abi Hazm narrated to us, 'Umar ibn 'Amir narrated to us, from Abu Ishaq, from al-Harith, from 'Ali, who said: The Messenger of Allah (ﷺ) said: "It has been pardoned for you regarding the Zakat of horses and slaves. But come forth with the Zakat of silver: from every forty dirhams, one dirham. And nothing shall be taken from you until it reaches two hundred dirhams. So when it reaches two hundred dirhams, then in it there are five dirhams."