Islam, one of the three major monotheistic faiths, was founded in Arabia by Muhammad between 610 and 632. There are an estimated 4.6 million Muslims in North America and 1.57 billion Muslims worldwide. Muhammad was born in A.D. 570 at Mecca and belonged to the Quraysh tribe, which was active in the caravan trade. At the age of 25 he joined the trade from Mecca to Syria in the employment of a rich widow, Khadija, whom he later married. Critical of the lax moral standards and polytheistic practices of the inhabitants of Mecca, he began to lead a contemplative life in the desert. In a dramatic religious vision, the angel Gabriel announced to Muhammad that he was to be a prophet. Encouraged by Khadija, he devoted himself to the reform of religion and society. Polytheism was to be abandoned. But leaders of the Quraysh generally rejected his teaching, and Muhammad gained only a small following and suffered persecution. He eventually fled Mecca. The Hegira (Hijra, meaning "emigration") of Muhammad from Mecca, where he was not honored, to Medina, where he was well received, occurred in 622 and marks the beginning of the Muslim era. After a number of military conflicts with Mecca, in 630 he marched on Mecca and conquered it. Muhammad died at Medina in 632. His grave there has since been a place of pilgrimage. Muhammad's followers, called Muslims, revered him as the prophet of Allah (God), the only God. Muslims consider Muhammad to be the last in the line of prophets that included Abraham and Jesus. Islam spread quickly, stretching from Spain in the west to India in the east within a century after the prophet's death. Sources of the Islamic faith are the Qur'an (Koran), regarded as the uncreated, eternal Word of God, and tradition (hadith) regarding sayings and deeds of the prophet. Islam means "surrender to the will of Allah," the all-powerful, who determines humanity's fate. Good deeds will be rewarded at the Last Judgment in paradise, and evil deeds will be punished in hell. The Five Pillars, or primary duties, of Islam are profession of faith; prayer, to be performed five times a day; almsgiving to the poor and the mosque (house of worship); fasting during daylight hours in the month of Ramadan; and pilgrimage to Mecca (the hajj) at least once in a Muslim's lifetime, if it is physically and financially possible. The pilgrimage includes homage to the ancient shrine of the Ka'aba, the most sacred site in Islam. Muslims gather for corporate worship on Fridays. Prayers and a sermon take place at the mosque, which is also a center for teaching of the Qur'an. The community leader, the imam, is considered a teacher and prayer leader. Islam succeeded in uniting an Arab world of separate tribes and castes, but disagreements concerning the succession of the prophet caused a division in Islam between two groups, Sunnis and Shi'ites. The Shi'ites rejected the first three successors to Muhammad as usurpers, claiming the fourth, Muhammad's son-in-law Ali, as the rightful leader. The Sunnis (from the word tradition), the largest division of Islam (today more than 87%), believe in the legitimacy of the first three successors. Among these, other sects arose (such as the conservative Wahhabi of Saudi Arabia), as well as different schools of theology. Another development within Islam, beginning in the eighth and ninth centuries, was Sufism, a form of mysticism. This movement was influential for many centuries and was instrumental in the spread of Islam in Asia and Africa. Islam has expanded greatly under Muhammad's successors. It is the principal religion of the Middle East, Asia, and the northern half of Africa.