From Sahih Muslim (Hadith 4798):
It has been narrated (through a still different chain of transmitters) on the Same authority (i. e. 'Arfaja) who said similarly-but adding:" Kill all of them." I heard the Messenger of Allah (ﷺ) say: When you are holding to one single man as your leader, you should kill who seeks to undermine your solidarity or disrupt your unity
Reference: Muslim 4798