Al-Masjid an-Nabawi or The Prophet's Mosque is the second most important religious building in the Islamic religion. This place of worship was built in 622 by the Prophet Muhammad upon his arrival in Medina. The mosque was expanded several times, so the tomb of the Prophet Muhammad became part of the complex. The tomb is known as the Green Dome and contains the remains of the Prophet Muhammad, his father-in-law, and companion Abu Bakr Abdullah ibn Uthman, Umar ibn al-Khattab, the heir to the head of the caliphate after Abu Bakr. The fourth tomb is reserved for the Prophet Isa (Jesus) because Muslims believe that he will come again and that he will be buried there. Al-Masjid an-Nabawi is one of the most visited religious sites among Muslims, as it is visited by almost all believers who come to Hajj.