Arabs of the Arabian Peninsula spread the word of the prophet Muhammad, calling for devotion to one God and for moral reform. From Muhammad's revelations came the Quran, the holy book of Islam, and the Five Pillars: faith, prayer, almsgiving, fasting, and pilgrimage to Makkah. After Muhammad's death Islam divided into two separate groups, the Sunnis and the Shiites. Caliphs, the successors of Muhammad, extended the Islamic Empire through a series of military victories over the Byzantine and Persian Empires. The Umayyad caliphs, based in Syria, carried Islam as far eastward as India and China as well as west to North Africa and parts of southern Europe. The Abbasid caliphs shifted the empire's center of power eastward to Iraq. Muslim scholars preserved Greek philosophy and made advances in mathematics, astronomy, geometry, and medicine.