Musnad Ahmad 30:3101

Waki’ narrated to us, from Ja’far ibn Burqan, from Yazid ibn al-Asamm, from Abu Hurairah, who said: The Messenger of Allah (ﷺ) said: "Tribulations will appear, *al-harj* (killing) will increase, and knowledge will be lifted." When Umar heard Abu Hurairah saying, "Knowledge will be lifted," Umar said: "As for it, it is not extracted from the chests of the scholars, but rather the scholars will pass away."