Musnad Ahmad 31:630

Muhammad ibn Mus'ab narrated to us, al-Awza'i narrated to us, from al-Zuhri, from Abu Salamah and al-Dahhak al-Mashriqi, from Abu Sa'id al-Khudri, who said: While the Messenger of Allah (peace be upon him) was one day distributing wealth, Dhu al-Khuwaysirah—a man from Banu Tamim—came to him and said: "O Muhammad, be just, for by Allah, you have not been just since today." The Prophet (peace be upon him) said: "By Allah, you will not find anyone after me who is more just to you than I am," three times. Umar said: "O Messenger of Allah, will you permit me to strike his neck?" He said: "No, for he has companions; one of you would consider his own prayer insignificant compared to their prayer, and his own fasting compared to their fasting. They pass through the religion just as an arrow passes through the game; its owner looks at its tip and sees nothing. Their sign is a man whose one hand is like a piece of flesh, or like a woman’s breast. They will emerge during a division among the people, and the party that is closer to Allah will kill them." Abu Sa'id said: "I bear witness that I heard this from the Messenger of Allah (peace be upon him), and I witnessed Ali when he killed them. He searched among the slain and found the one with the description that the Messenger of Allah (peace be upon him) had described."