Musnad Ahmad 30:431

Ibn Numayr narrated to us, from Hanzalah, who said: I heard Salim, who said: I heard Abu Hurayrah saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Knowledge will be seized, tribulations will appear, and *al-harj* will increase." It was said: O Messenger of Allah, and what is *al-harj*? He said: "Killing."