Musnad Ahmad 788:146

Abu Mu'awiyah narrated to us, from al-A'mash, from Shaqiq, from Abu Musa, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, ahead of you are days in which ignorance will descend, knowledge will be lifted, and *al-harj* will increase." They said: "O Messenger of Allah, and what is *al-harj*?" He said: "Killing."