كتاب الزكاة
12  
The Book of Zakat
(47)
Chapter: The Khawarij and their attributes
(47)
باب ذِكْرِ الْخَوَارِجِ وَصِفَاتِهِمْ ‏
Sahih Muslim 1065 b

Abu Sa'id al-Khudri reported that the Messenger of Allah (ﷺ) said:

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.
حَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا الْقَاسِمُ، - وَهُوَ ابْنُ الْفَضْلِ الْحُدَّانِيُّ - حَدَّثَنَا أَبُو نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ تَمْرُقُ مَارِقَةٌ عِنْدَ فُرْقَةٍ مِنَ الْمُسْلِمِينَ يَقْتُلُهَا أَوْلَى الطَّائِفَتَيْنِ بِالْحَقِّ ‏"‏ ‏.‏
Reference : Sahih Muslim 1065b
In-book reference : Book 12, Hadith 195
USC-MSA web (English) reference : Book 5, Hadith 2325
  (deprecated numbering scheme)