Musannaf Ibn Abi Shayba 38:145

Yahya ibn Adam narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Ubayd Allah ibn Abi Yazid, from Ibn ‘Abbas, that the Khawarij were mentioned in his presence. He mentioned their worship and their diligence, then he said: "They are not more diligent than the Jews and the Christians, yet they pray."