A--- Narrated Abu Dawud and others on the authority of Sa'id Ibn Jamhan after Safinah as saying: Allah's Messenger (Peace be upon him) says:

"The Caliphate after Prophethood will last for thirty years, then Allah grants kingship to whom He wills." (At-Tirmidhi, Abu Dawud, Ahmed and Al-Hakim)

This implies the period lasted by Abu Bakr, 'Umar, 'Uthman and `Ali, may Allah be pleased with them all. Abu Bakr stayed for two years and three months, 'Umar for ten years and six months, 'Uthman for twelve years, `Ali for four years and nine months, and in completion of the thirty years comes the Bai'ah (Pledge of allegiance) given for Al-Hasan Ibn 'Ali, that is six months.

The first Muslim king was Mu'awiyah (May Allah be pleased with him) who was the best among them all. Then, followed a severe and unjust kingship, up till the time when `Umar Ibn 'Abdel'Aziz came. Indeed, the Sunni Muslim scholars regarded him as the fifth Caliph as he followed their (the previous four ones) very steps.