Musnad Ahmad 907:207

Mu'ammal ibn Isma'il narrated to us, Hammad ibn Salamah narrated to us, Dawud ibn Abi Hind narrated to us, from al-Sha'bi, from Jabir ibn Samurah, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "There will be twelve caliphs for this nation."