Musnad Ahmad 31:620

'Affan narrated to us, Abu 'Awanah narrated to us, Qatadah narrated to us, from Abu Nadrah, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (ﷺ) said: "There will be two factions in my Ummah; a renegade group will emerge between them, and the one among the two that is closer to the truth will be responsible for killing them."