Musnad Ahmad 5:666

'Abdullah narrated to us, Al-'Abbas ibn al-Walid al-Narsi narrated to me, Abu 'Awanah narrated to us, Abu Ishaq narrated to us, 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 Zakat on] horses and slaves. So pay the Sadaqah (Zakat) on silver: from every forty dirhams, one dirham. There is nothing [due] on one hundred and ninety. But when it reaches two hundred, then in it are five dirhams."