The adhan is the Islamic call to prayer, a fixed sequence of Arabic phrases chanted aloud to summon Muslims to each of the five daily prayers, traditionally given by a muezzin from the minaret of a mosque, though today most often amplified through loudspeakers rather than delivered by unaided voice from the tower itself.
Unlike sung music built around a fixed melody that stays essentially the same performance to performance, the adhan is chanted according to a flexible melodic tradition in which the muezzin has real latitude in how he shapes the melodic line and ornamentation within the constraints of the fixed text and the conventions of correct pronunciation and pacing, so no two muezzins, or even the same muezzin on different days, necessarily deliver the call in identical melodic detail even though the words themselves do not change.
The adhan's text includes the takbir, proclaiming God as greatest, the shahada, the core testimony of faith, and calls to prayer and to success, arranged in a fixed sequence, with a specific additional phrase inserted into the dawn call reminding listeners that prayer is better than sleep, a well known textual variation unique to that time of day. A well trained muezzin's adhan is often specifically remembered and admired within a community, in much the way a cantor or choir might be within other traditions, even though the words he chants never vary from the same fixed text every mosque in the world uses for the call.