Musnad Ahmad 5:149

Surayj ibn al-Nu‘man narrated to us, Abu ‘Awanah narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have pardoned for you the horses and the slaves, so bring the Zakat of silver (al-riqqa): from every forty dirhams, one dirham. And there is nothing due on one hundred and ninety. When it reaches two hundred, then in it there are five dirhams."