A group would secede itself (from the Ummah) when there would be dissension among the Muslims. Out of the two groups who would be nearer the truth would kill them.
Sahih Muslim Book 12, Hadith 195
← Previous Hadith A man throws an arrow at the prey (or he said at the target), and sees at its iron head, but finds no sign (of blood there), or he sees at the lowest end, but would not see or find any sign (of blood there). He would then see into the grip but would not find (anything) sticking to it. Abu Sai'd then said: People of Iraq. it is you who have killed them.

Sahih Muslim Book 12, Hadith 194 Next Hadith → There would be two groups in my Ummah, and there would emerge another group (seceding itself from both of them), and the party nearer to the truth among the two would kill them (the group of the Khawarij).

Sahih Muslim Book 12, Hadith 196