First Mu’azzin in Islam:

‘Allaamah ibnul Atheer (rahimahullah) has mentioned that Hazrat Bilaal (radhiyallahu ‘anhu) was the first Mu’azzin in Islam. He would call out the azaan for Rasulullah (sallallahu ‘alaihi wasallam) while they were on safar (travelling) and while they were in Madinah Munawwarah.

(Usdul Ghaabah 1/237)

The Four Mu’azzins of Nabi (sallallahu ‘alaihi wasallam):

Haafiz ibnul Qayyim (rahimahullah) has recorded that four Sahaabah (radhiyallahu ‘anhum) enjoyed the privilege and honor of being the mu’azzins of Rasulullah (sallallahu ‘alaihi wasallam). Hazrat Bilaal (radhiyallahu ‘anhu) and Hazrat ibn umme Maktoom (radhiyallahu ‘anhu) were the mu’azzins of Madinah Munawwarah. Hazrat Sa‘d Al-Quraz (radhiyallahu ‘anhu), the freed slave of Hazrat ‘Ammaar bin Yaasir (radhiyallahu ‘anhu), was the mu’azzin of Qubaa’ and Hazrat Abu Mahzoorah (radhiyallahu ‘anhu) was the mu’azzin of Makkah Mukarramah.

(Zaadul Ma‘aad 1/120)

Appointed as Mu’azzin of Rasulullah (sallallahu ‘alaihi wasallam):

When Rasulullah (sallallahu ‘alaihi wasallam) made Hijrah (migrated) to Madinah Munawwarah, he constructed the musjid. After constructing the musjid, he consulted the Sahaabah (radhiyallahu ‘anhum) regarding the method to be adopted to call people for salaah. It was the burning desire within the heart of Rasulullah (sallallahu ‘alaihi wasallam) that all the Sahaabah (radhiyallahu ‘anhum) congregate and perform their salaah together in the musjid. Rasulullah (sallallahu ‘alaihi wasallam) was neither pleased with the Sahaabah (radhiyallahu ‘anhum) performing their salaah in the musjid at different times nor in their homes or at other places.

The Sahaabah (radhiyallahu ‘anhum) presented various suggestions in regard to how people could be called for salaah. Some of the suggestions of the Sahaabah (radhiyallahu ‘anhum) were that a fire be lit or a flag be hoisted. On seeing the fluttering flag or the flames and smoke of the fire, people would understand that it is the time of salaah and thereby inform others to come to the musjid for salaah. Other suggestions were that a horn be sounded or that the Naqoos (two sticks) be struck upon each other to alert people that it is the time for salaah. Rasulullah (sallallahu ‘alaihi wasallam) was not pleased with these opinions. Rasulullah (sallallahu ‘alaihi wasallam) did not want his Ummah to emulate the Christians, Jews and Kuffaar in aspects of their deen or their worldly life. If the Muslims were to adopt these methods, it would result in them resembling the disbelievers in their deen and furthermore, it would lead to confusion coming about in the salaah times as the disbelievers were calling people at other times through these same methods. No conclusion was reached in that gathering and the matter was thus left undecided.