كتاب الإمارة
33
The Book on Government
(14)
Chapter: The ruling on one who seeks to divide the muslims when they are united
(14)
باب حُكْمِ مَنْ فَرَّقَ أَمْرَ الْمُسْلِمِينَ وَهُوَ مُجْتَمِعٌ ‏‏

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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1852 c
In-book reference : Book 33, Hadith 95
USC-MSA web (English) reference : Book 20, Hadith 4567
  (deprecated numbering scheme)