Al-Matalib al-'Aliyah 4437 — The virtue of one who kills

Asim bin Kulayb narrates from his father that the people's places of gathering used to be the mosques, until they returned from (the Battle of) Siffin and declared themselves free of the arbitration; then the people began to regard (the importance of the mosques) lightly and to sit in the streets and lanes exchanging news. Meanwhile, while we were sitting with Sayyiduna Ali (may Allah be pleased with him), a man suddenly stood up and said: "Give me permission to speak," but he (may Allah be pleased with him) was busy with some task of his. We asked the man: "What do you want to ask the Commander of the Believers about?" He said: "I went for Umrah and visited Sayyidah Aishah (may Allah be pleased with her). She asked who those people are who have gone out (in rebellion) from your side and are called Harura' (the Khawarij). Then she asked: 'Were you present at the time of their destruction (on the battlefield)? Know well! If Ali bin Abi Talib wished, he could tell you their story.'" When Sayyiduna Ali (may Allah be pleased with him) was free from his task he asked: "Where is that man?" The man told him the whole story, and Sayyiduna Ali (may Allah be pleased with him) said aloud 'La ilaha illallah' (There is no god worthy of worship except Allah) and 'Allahu akbar' (Allah is the Greatest). Then he said: "I came to the Messenger of Allah (peace and blessings be upon him) when there was no one with him except Sayyidah Aishah (may Allah be pleased with her), and the Prophet (peace and blessings be upon him) said to me: 'O son of Abu Talib! How will you be when such and such a people (appear)?' I said: 'Allah and His Messenger know best.' He (peace and blessings be upon him) said: 'A people will come out from the direction of the east who will recite the Qur'an, but it will not go beyond their throats; they will pass out of the religion as an arrow passes through the game. Among them will be a man whose hand will be deficient (like a lump of flesh), as though his hand were the breast of an Abyssinian woman.'" Then Sayyiduna Ali (may Allah be pleased with him) said (to the people present there): "I ask you by Allah, had I not told you earlier that this man would be among them? But you said that he was not among them, then you brought him to me, dragging him?" The people said: "O Allah (be witness), yes (so it happened)." Then Sayyiduna Ali (may Allah be pleased with him) again said aloud 'La ilaha illallah' (There is no god worthy of worship except Allah) and 'Allahu akbar' (Allah is the Greatest).
The chain of this hadith is hasan (good), and the basis of its essential parts is found in the two Sahihs (al-Bukhari and Muslim); Imam al-Hakim declared it sahih (authentic) according to his condition.