Musannaf Ibn Abi Shayba 38:186

Yahya ibn Adam narrated to us, Mufaddal ibn Muhalhal narrated to us, from al-Shaybani, from Qays ibn Muslim, from Tariq ibn Shihab, who said: I was with Ali when he was asked about the people of the river (the Khawarij): "Are they polytheists?" He said: "From polytheism they fled." It was said: "Are they hypocrites?" He said: "Indeed, the hypocrites do not remember Allah except a little." It was said: "Then what are they?" He said: "A people who committed transgression against us."