Musannaf Ibn Abi Shayba 37:481

Waki’ narrated to us, from Ja’far ibn Burqan, from Yazid ibn al-Asamm, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Tribulations will increase, and al-harj will increase." We said: "And what is al-harj?" He said: "Killing. And knowledge will decrease." He said: "Know that it is not snatched away from the chests of men, but the scholars will be taken away."