Sahih Muslim 1065 a
Abu Sa'id al-Khudri said that the Messenger of Allah (ﷺ) made a mention of a sect that would be among his Ummah which would emerge out of the dissension of the people. Their distinctive mark would be shaven heads. They would be the worst creatures or the worst of the creatures. The group who would be nearer to the truth out of the two would kill them. The Apostle of Allah (ﷺ) gave an example (to give their description) or he said:
A man throws an arrow at the prey (or he said at the target), and sees at its iron head, but finds no sign (of blood there), or he sees at the lowest end, but would not see or find any sign (of blood there). He would then see into the grip but would not find (anything) sticking to it. Abu Sai'd then said: People of Iraq. it is you who have killed them.
وَحَدَّثَنِي مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، عَنْ سُلَيْمَانَ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم ذَكَرَ قَوْمًا يَكُونُونَ فِي أُمَّتِهِ يَخْرُجُونَ فِي فُرْقَةٍ مِنَ النَّاسِ سِيمَاهُمُ التَّحَالُقُ قَالَ ‏"‏ هُمْ شَرُّ الْخَلْقِ - أَوْ مِنْ أَشَرِّ الْخَلْقِ - يَقْتُلُهُمْ أَدْنَى الطَّائِفَتَيْنِ إِلَى الْحَقِّ ‏"‏ ‏.‏ قَالَ فَضَرَبَ النَّبِيُّ صلى الله عليه وسلم لَهُمْ مَثَلاً أَوْ قَالَ قَوْلاً ‏"‏ الرَّجُلُ يَرْمِي الرَّمِيَّةَ - أَوْ قَالَ الْغَرَضَ - فَيَنْظُرُ فِي النَّصْلِ فَلاَ يَرَى بَصِيرَةً وَيَنْظُرُ فِي النَّضِيِّ فَلاَ يَرَى بَصِيرَةً وَيَنْظُرُ فِي الْفُوقِ فَلاَ يَرَى بَصِيرَةً ‏"‏ ‏.‏ قَالَ قَالَ أَبُو سَعِيدٍ وَأَنْتُمْ قَتَلْتُمُوهُمْ يَا أَهْلَ الْعِرَاقِ ‏.‏