Musnad Ahmad 5:84

Fadl ibn Dukayn narrated to us, Yasin al-Ijli narrated to us, from Ibrahim ibn Muhammad ibn al-Hanafiyyah, from his father, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Mahdi is from us, the People of the House (Ahl al-Bayt). Allah will rectify him in a single night."