The spread of Islam to the North Eastern tip of the Arabian Peninsula is thought to have followed directly from a letter sent by the Islamic Prophet, Muhammad, to the rulers of Oman in 630 CE, nine years after the hijrah. This led to a group of rulers travelling to Medina, converting to Islam and subsequently driving a successful uprising against the unpopular Sasanids, who dominated the Northern coasts at the time. Following the death of Muhammad, the new Islamic communities south of the Persian Gulf threatened to disintegrate, with insurrections against the Muslim leaders. The Caliph Abu Bakr sent an army from the capital Medina which completed its reconquest of the territory (the Ridda Wars) with the Battle of Dibba in which 10,000 lives are thought to have been lost. This assured the integrity of the Caliphate and the unification of the Arabian Peninsula under the newly emerging Rashidun Caliphate.