Musnad Ahmad 31:458

Waki' narrated to us, from al-Qasim ibn al-Fadl, Abu Nadra al-'Abdi narrated to us, from Abu Sa'id, who said: The Messenger of Allah (ﷺ) said: "A renegade group will emerge at a time of division among the Muslims; the group that is closer to the truth will kill them."