Musannaf Ibn Abi Shayba 27:325

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from 'Abdullah ibn Salamah, that 'Ali said: "Shall I not inform you of the best of this nation after the Messenger of Allah, may Allah bless him and grant him peace? Abu Bakr and 'Umar ibn al-Khattab."