Narrated Abu Sa`id Al-Khudri:

While we were with Allah's Messenger (ﷺ) who was distributing (i.e. some property), there came Dhu-l- Khuwaisira, a man from the tribe of Bani Tamim and said, "O Allah's Messenger (ﷺ)! Do Justice." The Prophet said, "Woe to you! Who could do justice if I did not? I would be a desperate loser if I did not do justice." `Umar said, "O Allah's Messenger (ﷺ)! Allow me to chop his head off." The Prophet (ﷺ) said, "Leave him, for he has companions who pray and fast in such a way that you will consider your fasting negligible in comparison to theirs. They recite Qur'an but it does not go beyond their throats (i.e. they do not act on it) and they will desert Islam as an arrow goes through a victim's body, so that the hunter, on looking at the arrow's blade, would see nothing on it; he would look at its Risaf and see nothing: he would look at its Na,di and see nothing, and he would look at its Qudhadh ( 1 ) and see nothing (neither meat nor blood), for the arrow has been too fast even for the blood and excretions to smear. The sign by which they will be recognized is that among them there will be a black man, one of whose arms will resemble a woman's breast or a lump of meat moving loosely. Those people will appear when there will be differences amongst the people." I testify that I heard this narration from Allah's Messenger (ﷺ) and I testify that `Ali bin Abi Talib fought with such people, and I was in his company. He ordered that the man (described by the Prophet (ﷺ) ) should be looked for. The man was brought and I looked at him and noticed that he looked exactly as the Prophet (ﷺ) had described him.