Musnad Ahmad 1255
It was narrated that Tariq bin Ziyad said:
‘Ali marched to an-Nahrawan and killed the Khawarij. Then he said: Look, for     the Prophet (ﷺ) said: `There will come a people who will speak the word of truth but it will not go any further than their throats; they will pass out of Islam as the arrow passes out of the prey. Their sign - or among them - will be a black man with a deformed arm, on which there are black hairs. If he is among them, then you will have killed the worst of people; if he is not among them, then you will have killed the best of people.” Then we found the one with the deformity, and we fell down in prostration and ‘Ali fell down in prostration with us.       .
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ إِبْرَاهِيمَ بْنِ عَبْدِ الْأَعْلَى، عَنْ طَارِقِ بْنِ زِيَادٍ، قَالَ سَارَ عَلِيٌّ إِلَى النَّهْرَوَانِ فَقَتَلَ الْخَوَارِجَ فَقَالَ اطْلُبُوا فَإِنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ سَيَجِيءُ قَوْمٌ يَتَكَلَّمُونَ بِكَلِمَةِ الْحَقِّ لَا يُجَاوِزُ حُلُوقَهُمْ يَمْرُقُونَ مِنْ الْإِسْلَامِ كَمَا يَمْرُقُ السَّهْمُ مِنْ الرَّمِيَّةِ سِيمَاهُمْ أَوْ فِيهِمْ رَجُلٌ أَسْوَدُ مُخْدَجُ الْيَدِ فِي يَدِهِ شَعَرَاتٌ سُودٌ إِنْ كَانَ فِيهِمْ فَقَدْ قَتَلْتُمْ شَرَّ النَّاسِ وَإِنْ لَمْ يَكُنْ فِيهِمْ فَقَدْ قَتَلْتُمْ خَيْرَ النَّاسِ قَالَ ثُمَّ إِنَّا وَجَدْنَا الْمُخْدَجَ قَالَ فَخَرَرْنَا سُجُودًا وَخَرَّ عَلِيٌّ سَاجِدًا مَعَنَا‏.‏