Musnad Ahmad 5:359

'Abdullah narrated to us, Wahb ibn Baqiyyah al-Wasiti narrated to me, Khalid informed us, from 'Ata'—meaning Ibn al-Sa'ib—from 'Abd Khayr, from 'Ali, who said: "Shall I not inform you of the best of this nation after its Prophet? Abu Bakr. And the best of it after Abu Bakr: 'Umar. Then Allah places goodness wherever He wills."