One of the five pillars of Islam is the ritual prayer (salat) which is performed by Muslims around the world five times a day.

How Muslims know that the prayer time has come is by the adhan, or the call to prayer that is recited from the minaret of a mosque.

The first adhan was recited in the first Hijri year, or 622 CE. Bilal was the first Muslim to deliver it, and ever since, it has been an eternal melodic call, echoed by the world.