Mishkat al-Masabih 16 Retaliation كتاب القصاص 83

وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «يَكُونُ أُمَّتِي فِرْقَتَيْنِ فَيَخْرُجُ مِنْ بَيْنِهِمَا مَارِقَةٌ يَلِي قَتْلَهُمْ أولاهم بِالْحَقِّ» . رَوَاهُ مُسلم

Abu Sa’id al-Khudri reported God’s Messenger as saying, “My people will be in two sections from among whom a seceding party will go out, and those who are nearest to the truth will undertake the killing of them.” Muslim transmitted it.