Musnad Abu Ya'la al-Mawsili 160:250

Muhammad ibn Bakkar narrated to us, al-Walid ibn Abi Thawr narrated to us, from Asim ibn Bahdalah, from Ziyad ibn Qays, from Abu Hurayrah, from the Messenger of Allah (may Allah bless him and grant him peace), who said: "Woe to the Arabs from an evil that has drawn near. Knowledge will decrease, and al-harj will increase." He said: I said, "What is the increase of al-harj, O Messenger of Allah?" He said: "Killing, killing."