Musannaf Ibn Abi Shayba 38:160

Waki’ narrated to us, he said: Al-A’mash narrated to us, from ‘Amr ibn Murrah, from ‘Abdullah ibn al-Harith, from a man from Banu Nasr ibn Mu’awiyah, who said: We were with ‘Ali, and they mentioned the people of the river (the Khawarij), and a man cursed them. ‘Ali said: Do not curse them. But if they rise against a just Imam, then fight them; and if they rise against an unjust Imam, then do not fight them, for they have an argument in that.