More than 2 million Muslims began the annual hajj pilgrimage at first light Sunday in Saudi Arabia. 

The five-day hajj pilgrimage represents one of the world's biggest gatherings every year, and is required of all able-bodied Muslims once in their life, according to CBS News.

The hajj offers pilgrims an opportunity to feel closer to God. 

Muslims believe the hajj retraces the footsteps of the Prophet Muhammad, as well as those of the prophets Ibrahim and Ismail — Abraham and Ishmael in the Bible.  Muslims believe God stayed the hand of Ibrahim after commanding him to sacrifice his son, Ismail. In the Christian and Jewish version of the story, Abraham is ordered to kill his other son, Isaac.

The Kaaba represents the metaphorical house of God and the oneness of God.

Muslims circle the Kaaba counter-clockwise seven times while reciting supplications to God, then walk between the two hills traveled by Hagar, Ibrahim's wife. Mecca's Grand Mosque, the world's largest, encompasses the Kaaba and the two hills.