Musnad Ahmad

Zaid bin Wahb narrates that when the Khawarij rebelled at Nahrawan, Sayyiduna Ali (may Allah be pleased with him) stood among his companions and said: "These people have shed blood unjustly, have plundered people's animals, and they are your nearest enemies. Therefore, in my opinion, you should march towards your enemies. I fear that these people may attack you from behind. And I heard the Prophet (peace and blessings be upon him) say that a group will appear in my Ummah whose prayers will seem insignificant compared to your prayers, whose fasts will seem insignificant compared to your fasts, whose recitation will seem insignificant compared to your recitation. They will recite the Qur'an thinking that they are earning reward from it, but it will be a cause of punishment for them, because the Qur'an will not go below their throats. They will go out of Islam just as an arrow passes through its prey. And their sign will be that among them there will be a man who will have an arm but no elbow, and on his hand there will be a mark like the nipple of a woman's breast, around which will be a cluster of white hairs. If any army that attacks them were to know the reward that has been promised about them by their Prophet, they would rely solely on that. Therefore, set out in the name of Allah." After this, the narrator mentioned the complete hadith.