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Sunni Islam (/ ˈ s uː n i, ˈ s ʊ n i /) is the largest branch of Islam, followed by 85–90% of the world's Muslims, and simultaneously the largest religious denomination in the world. Its name comes from the word Sunnah, referring to the tradition of Muhammad. The differences between Sunni and Shia Muslims arose from a disagreement over the succession to Muhammad and subsequently acquired broader political significance, as well as theological and juridical dimensions. According to Sunni traditions, Muhammad left no successor and the participants of the Saqifah event appointed Abu Bakr as the next-in-line (the first caliph). This contrasts with the Shia view, which holds that Muhammad appointed his son-in-law and cousin Ali ibn Abi Talib as his successor.

The adherents of Sunni Islam are referred to in Arabic as ahl as-sunnah wa l-jamāʻah ("the people of the Sunnah and the community") or ahl as-Sunnah for short. In English, its doctrines and practices are sometimes called Sunnism. Sunni Islam is sometimes referred to as " orthodox Islam ", though some scholars view this as inappropriate, and many non-Sunnis may find this offensive.

The Quran, together with hadith (especially the Six Books) and ijma (juristic consensus), form the basis of all traditional jurisprudence within Sunni Islam. Sharia rulings are derived from these basic sources, in conjunction with analogical reasoning, consideration of public welfare and juristic discretion, using the principles of jurisprudence developed by the traditional legal schools. In matters of creed, the Sunni tradition upholds the six pillars of iman (faith) and comprises the Ash'ari and Maturidi schools of kalam (theology) as well as the textualist Athari school. Sunnis regard the first four caliphs Abu Bakr (r. 632–634), Umar (r. 634–644), Uthman (r. 644–656) and Ali (r. 656–661) as rashidun (rightly-guided) and revere the sahaba, tabi'in and tabi al-tabi'in as the salaf (predecessors).