Musnad Ahmad 5:533

Waki’ narrated to us, from Sufyan, 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. But bring the quarter of the tenth (2.5%)—from every forty dirhams, one dirham."