Musnad Ahmad 31:285

Waki’ narrated to us, Qasim ibn al-Fadl narrated to us, Abu Nadra narrated to us, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A renegade group will emerge at a time of division among the Muslims, and the group that is closer to the truth will kill them."