Musannaf Ibn Abi Shayba 37:533

Ibn ‘Uyaynah narrated to us, from ‘Amr, from Abu Ma’bad, from Ibn ‘Abbas, who said: The days and nights will not pass until a youth from us, the People of the House (Ahl al-Bayt), takes charge, whom the tribulations have not clothed, and who has not clothed himself in them. He said: We said, "O Abu al-‘Abbas, your elders are incapable of it, yet your youth will attain it?" He said: "It is the command of Allah; He gives it to whom He wills."