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)