Musnad Ahmad 28:311

Waki’ narrated to us, Hisham informed us, from his father, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah does not take away knowledge by snatching it away from the people, but He takes away knowledge by the death of the scholars, until when no scholar remains, the people will take ignorant leaders. They will be asked, and they will give fatwas without knowledge, so they will go astray and lead others astray."