Najaf is considered sacred by both Shi'a and Sunni Muslims. Najaf is renowned as the site of the tomb of Al? ibn Ab? T?lib also known as "Im?m Al?"the First Imam of the Shiites, the cousin and son-in-law of the prophet Muhammad whom the Shi'a consider to be the righteous caliph . Sunnis consider Ali the fourth Rashidun (rightly guided Caliphs). The city is now a great center of pilgrimage from throughout the Shi'a Islamic world. It is estimated that only Mecca and Medina receive more Muslim pilgrims. As the burial site of Shi'a Islam's second most important figure, the Imam Ali Mosque is considered by Shiites as the third holiest Islamic site.