The Book of Zakat
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 " .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
From Sahih Muslim, whose reports the scholars accept as authentic.