Hadithcore
Mishkat al-Masabih, 3678

He told that he heard God’s Messenger say, “If anyone comes to you when you are united under one man desiring to split you or cause parties in your community, kill him.” Muslim transmitted it.

وَعَنْهُ قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «مَنْ أَتَاكُمْ وَأَمْرُكُمْ جَمِيعٌ عَلَى رَجُلٍ وَاحِدٍ يُرِيدُ أَنْ يَشُقَّ عَصَاكُمْ أوْ يُفرِّقَ جماعتكم فَاقْتُلُوهُ» . رَوَاهُ مُسلم

References1 variant
In-Book Reference
Book 18, Hadith 18