Musnad Ahmad 949:1

Bahz narrated to us, Hammad ibn Salamah narrated to us, Sa’id ibn Jumhan narrated to us—and [likewise] ‘Abd al-Samad narrated to me, Hammad narrated to me, Sa’id ibn Jumhan narrated to me—from Safinah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The Caliphate will be thirty years, then after that there will be kingship." Safinah said: "Hold [the count]: the Caliphate of Abu Bakr was two years, the Caliphate of ‘Umar was ten years, the Caliphate of ‘Uthman was twelve years, and the Caliphate of ‘Ali was six years."