Sahih Muslim 1852
Sahih Muslim · Book 33: The Book on Government · The ruling on one who seeks to divide the muslims when they are united
It has been narrated (through a still different chain of transmitters) on the Same authority (i. e. 'Arfaja) who said similarly-but adding:
وَحَدَّثَنِي عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا يُونُسُ بْنُ أَبِي يَعْفُورٍ ، عَنْ أَبِيهِ ، عَنْ عَرْفَجَةَ ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " مَنْ أَتَاكُمْ وَأَمْرُكُمْ جَمِيعٌ عَلَى رَجُلٍ وَاحِدٍ يُرِيدُ أَنْ يَشُقَّ عَصَاكُمْ أَوْ يُفَرِّقَ جَمَاعَتَكُمْ فَاقْتُلُوهُ " .
" 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.