Musnad Ahmad 5:134

Abdullah narrated to us, Muhammad ibn Abi Bakr al-Muqaddami narrated to me, Fudayl ibn Sulayman—meaning al-Numayri—narrated to us, Muhammad ibn Abi Yahya narrated to us, from Iyas ibn Amr al-Aslami, from Ali ibn Abi Talib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There will be disagreement after me, or an affair. So if you are able to be the peace, then do so."