Musannaf Ibn Abi Shayba 38:166

Muhammad ibn Bishr narrated to us, he said: Ubayd Allah ibn al-Walid narrated to us, from al-Hasan, from Ubayd ibn al-Hasan, he said: The Khawarij said to Umar ibn Abd al-Aziz: "Do you intend to conduct yourself among us according to the conduct of Umar ibn al-Khattab?" He said: "What is the matter with them? May Allah fight them! By Allah, I have done nothing more than to take the Messenger of Allah, may Allah bless him and grant him peace, as an Imam."