Musannaf Ibn Abi Shayba 27:415

Abu Usama narrated to us, he said: Hisham narrated to us, from Muhammad ibn Sirin, from Uqbah ibn Aws al-Sadusi, from Abdullah ibn Umar, who said: "There will be twelve caliphs in this nation. Abu Bakr—you have hit upon his name. And Umar ibn al-Khattab, a horn of iron—you have hit upon his name. And Uthman ibn Affan, the possessor of the two lights, who was given two portions of His mercy and was killed oppressed—you have hit upon his name."