Musnad Abu Ya'la al-Mawsili 42:273

Zuhayr narrated to us, Abu 'Amir al-'Aqadi narrated to us, from al-Qasim ibn al-Fadl, from Abu Nadrah, from Abu Sa'id, that the Prophet (peace and blessings of Allah be upon him) said: "A renegade group will emerge during a division among the Muslims. The group that is closer to the truth will kill them."