Musnad Ahmad 5:488

Abdullah narrated to us, Abu Salih al-Hakam ibn Musa narrated to us, Shihab ibn Khirash narrated to us, Yunus ibn Khabbab informed me, from al-Musayyab ibn Abd Khayr, from Abd Khayr, who said: I heard Ali saying: "Indeed, the best of this nation after its Prophet is Abu Bakr, then Umar, may Allah be pleased with them both."