Musnad Ahmad 949:0

The Hadith of Abu ‘Abd al-Rahman Safinah, the freed slave of the Messenger of Allah, may Allah bless him and grant him peace: Safinah said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The Caliphate in my nation will last for thirty years, then there will be kingship after that." Then Safinah said to me: "Calculate the Caliphate of Abu Bakr, then the Caliphate of ‘Umar, then the Caliphate of ‘Uthman, then the Caliphate of ‘Ali." He said: We found them to be thirty years.