Sunan Ibn Majah, Chapter: The Chapters on Tribulations

that the Messenger of Allah (saw) said: that the Messenger of Allah (saw) said:"Before the Hour comes there will be Harj." I said: "O Messenger of Allah, what is Harj?" He said: "Killing." Some of the Muslims said: "O Messenger of Allah, now we kill such and such a number of idolaters in one year." The Messenger of Allah (saw) said: "That will not be like killing the idolaters, rather you will kill one another, until a man will kill his neighbor and son of the cousin and a relative." Some of the people said: "O Messenger of Allah, will we be in our right minds that day?" The Messenger of Allah (saw) said: "No, reason will be taken away from most of the people at that time, and there will be left the insignificant people who have no reason."