Musannaf Ibn Abi Shayba 38:152

Jarir narrated to us, from Mughira, who said: Umar ibn Abd al-Aziz debated the Khawarij, and some of those who were among them returned [from their position], while a faction of them refused to return. So Umar sent a man with cavalry and ordered him to encamp where they encamp, and not to provoke them or agitate them. If they kill and cause corruption in the land, then overpower them and fight them. But if they do not kill and do not cause corruption in the land, then leave them to travel.