One of Islam's two holiest cities (with Mecca being the holiest), Medina is the site for millions of pilgrims making their way to Saudi Arabia every year (as it is a tradition to visit the Prophet Muhammad's grave after visiting Mecca). At its heart is the Al-Masjid an-Nabawi or the Prophet's Mosque. Built by the Prophet Muhammad, it's also his burial site (under the famous Green Dome, where Rashidun Caliphs Abu Bakr and Umar are also buried). The Masjid Quba is another holy site, as it very well may be the first mosque in the world (Muhammad himself is said to have laid the first stones when first arriving in Medina from Mecca). The Masjid al-Qiblatayn is said to be the mosque where Muhammad received instruction to change the direction of prayer from Jerusalem to Mecca. At the same time, Uhud Mountain was the location for the second great battle between Islam and non-believers.