Musnad Ahmad 30:3795

Hassan and Hashim narrated to us, they said: Shayban narrated to us, from Asim, from Ziyad ibn Qays, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Woe to the Arabs from an evil that has drawn near. Knowledge will decrease, and *al-harj* will increase." I said: "O Messenger of Allah, and what is *al-harj*?" He said: "Killing."