Sunan Ibn Majah 176

Abu Ghalib narrated that Abu Umamah said:"(The Khawarij) are the worst of the slain who are killed under heaven, and the best of the slain are those who were killed by them. Those (Khawarij) are the dogs of Hell. Those people were Muslims but they became disbelievers." I said: "O Abu Umamah, is that your opinion?" He said: "Rather I heard it from the Messenger of Allah