Ten years after the migration (Hijrah), the Islamic prophet Muhammad ordered his followers to call upon people everywhere to join him in his last pilgrimage. Islamic scholars believe more than seventy thousand people followed Muhammad on his way to Mecca, where, on the fourth day of the month of Dhu'l-Hijjah, there were more than one hundred thousand Muslims present for his entry into the city. While returning from this pilgrimage, on 18 Dhu'l-Hijjah 10 AH (March 632 CE) at an area known as Ghadir Khumm, Muhammad delivered a well-known sermon during which he called up his cousin and son-in-law Ali ibn Abi Talib and declared, "to whomsoever I am Mawla, Ali is also their Mawla".it was the official designation of Ali as Muhammad's successor. As a result, the date of the sermon is considered to be one of the foundational events of Shia Islam, with the anniversary becoming one of its most important annual celebrations as "Eid al-Ghadir".