Shaykhul Islaam Ibn Taymiyyah (may Allaah have mercy upon him said): "They (Ahlus Sunnah Wal-Jamaa'ah) accept what has been reported continuously from the Prince of the Believers, 'Ali Ibn Abi Talib (may Allah be pleased with him), and from others, that the best men of this 'Ummah after its Prophet are: Abu Bakr; then 'Umar; third, 'Uthman, and fourth, 'Ali Ibn Abi Talib (may Allah be pleased with them all). All Traditions have indicated, and all Sahabah (may Allah be pleased with them all) have agreed upon giving priority to 'Uthman out of regard for his allegiance (al-Bai'ah), although some of the people of the Sunnah are disputing over whether 'Uthman or 'Ali (may Allah be pleased with both of them) has the priority, after they (the people of the Sunnah) had agreed upon giving priority to Abu Bakr and 'Umar. Some people gave the priority to 'Uthman and kept silent and considered 'Ali to be the fourth. However, some people preferred 'Ali. And some remained neutral. But the people of the Sunnah settled on preferring 'Uthman, even though this matter - the matter of 'Uthman and 'Ali - is not of the fundamentals. The majority of the people of the Sunnah do not consider disagreeing in this matter as being misled. Rather, it is in the matter of the "Question of the Caliphate" where they consider the disagreeing person to be misled.
Ahl as-Sunnah believe that the Caliph after the Messenger of Allah (peace be upon him) is Abu Bakr; then 'Umar; then 'Uthman, then 'Ali, and that whoever contests the Caliphate of any one of these Imams is indeed more lost than an ass." ('Aqeedatul-Waasitiyyah)
The Messenger of Allaah (sallallaahu 'alaihi wa sallam) said, 'Every Prophet has two ministers (wazeeran) from the inhabitants of the Heavens, and two ministers from those of the Earth. As for my two ministers of the Heavens, they are Jibreel (AS) and Mika'eel (AS), and for my two ministers from those of the earth are Abu Bakr (RA) and Umar (RA).' (Tirmidhi, al-Manaaqib, 3613). It is related from Ibn Abbaas (RA) that a hypocrite had a dispute with a Jew. The Jew summoned him to the Messenger of Allaah (sallallaahu 'alaihi wa sallam), and the hypocrite summoned him to Ka'b ibn al-Ashraf. They [finally] took the case to the Messenger of Allaah (sallallaahu 'alaihi wa sallam), who passed a ruling in favour of the Jew. The hypocrite was not pleased and insisted that they go to Umar (RA) for a ruling. The Jew told Umar (RA) that the Messenger of Allaah (sallallaahu 'alaihi wa sallam) had already ruled in his favour and the hypocrite had not been pleased. He had insisted that they come to 'Umar (RA). 'Umar (RA) asked the hypocrite whether that was true, and he replied that it was. 'Umar (RA) instructed them to remain where they were until he returned. He went inside, picked up his sword, and came out and beheaded the hypocrite. He then said, 'This is my ruling for the person who is not satisfied with the ruling of Allaah and His Messenger (sallallaahu 'alaihi wa sallam).' Jibreel (AS) states, 'Umar (RA) differentiated between the truth and untruth, so he was named the Differentiator (al-Farooq).' [This is as stated in the Tafseer of Qadi Baywaawi (ra)]