Musnad Ahmad 5:420

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from Abu Ishaq, from al-Harith, 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. And regarding silver (al-riqah), there is a quarter of a tenth (2.5%) upon it."