Islam's spiritual center is Mecca, a city in Saudi Arabia. It is the holiest city in Islam and the birthplace of the Prophet Muhammad. Muslims from all over the world travel to Mecca to perform the Hajj, an annual pilgrimage that is one of the five pillars of Islam. The Kaaba, a cube-shaped building located in the center of the Grand Mosque, is the most sacred site in Islam and is believed to have been built by Abraham and his son Ishmael.