Musnad Ahmad 5:144

Abdullah narrated to us, Ahmad ibn Jamil Abu Yusuf narrated to us, Yahya ibn Abd al-Malik ibn Humayd ibn Abi Ghaniyah informed us, from Abd al-Malik ibn Abi Sulayman, from Salamah ibn Kuhayl, from Zayd ibn Wahb, who said: When the Khawarij emerged at Nahrawan, Ali stood among his companions and said: "These people have shed forbidden blood and raided the livestock of the people, and they are the closest enemy to you. If you march toward your enemy, I fear that these people will replace you in your rear. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'A group will emerge from my Ummah; your prayer is nothing compared to their prayer, your fasting is nothing compared to their fasting, and your recitation is nothing compared to their recitation. They recite the Quran, thinking it is for them, while it is against them. It does not pass beyond their throats. They pass through Islam just as an arrow passes through the game. The sign of that is that among them is a man who has an upper arm but no forearm, upon which is something like a nipple, with white hairs on it. If the army that strikes them knew what has been promised to them upon the tongue of their Prophet, they would rely [too much] on that deed. So, march in the name of Allah.'" He then mentioned the rest of the hadith in full.