‘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)
Azaan on the Ka‘bah:
On the occasion of Fat-he Makkah (the Conquest of Makkah), Rasulullah (sallallahu ‘alaihi wasallam) entered the Ka‘bah with Hazrat Bilaal (radhiyallahu ‘anhu) and Hazrat Usaamah (radhiyallahu ‘anhu). The musjid was filled with the Quraish who had formed rows and were watching Rasulullah (sallallahu ‘alaihi wasallam) to see what he was going to do. Other people of the Quraish were watching from the mountain tops around Makkah Mukarramah and some of the eminent personalities of the Quraish had chosen to flee and went into hiding. It was at that juncture that Rasulullah (sallallahu ‘alaihi wasallam) instructed Hazrat Bilaal (radhiyallahu ‘anhu) to ascend the Ka‘bah and call out the azaan for Zuhr salaah.
When Hazrat Bilaal (radhiyallahu ‘anhu) ascended the Ka‘bah to call out the azaan , ‘Attaab bin Aseed, Haarith bin Hishaam, Suhail bin ‘Amr and Abu Sufyaan (radhiyallahu ‘anhu) were among the Quraish seated in the courtyard around the Ka‘bah. On seeing Hazrat Bilaal (radhiyallahu ‘anhu) calling out the azaan, ‘Attaab bin Aseed commented, “Allah has honored my father, Aseed, by taking his life before this day, saving him from hearing the call of the azaan today. Had he been alive and heard this call, he would have been greatly angered.” Haarith bin Hishaam remarked, “By Allah! If I regarded this religion to be true and regarded him to be calling to the truth then I would have followed him.” Haarith, however, also made the following derogatory remark, “Could Muhammed (sallallahu ‘alaihi wasallam) not find anyone to call out the azaan besides this black crow?” Suhail bin ‘Amr said, “If Allah dislikes something, he will change it.” Hazrat Abu Sufyaan (radhiyallahu ‘anhu) said, “I will not say anything as I fear that these stones will inform Nabi (sallallahu ‘alaihi wasallam) of what I said.”
As soon as they had spoken, Hazrat Jibraa’eel (‘alaihis salaam) came to Nabi (sallallahu ‘alaihi wasallam) and informed him of what they had uttered. Nabi (sallallahu ‘alaihi wasallam) therefore called them and said, “I have been informed of the remarks you made.” after which he told each of them the exact remark that he had made. Haarith and ‘Attaab were amazed and immediately accepted Islam exclaiming, “We testify that you are the Rasul of Allah as you have informed us of our secret remarks! There was nobody else with us who could have possibly told you of what we said.”
The following verse of the Qur’aan Al-Kareem was then revealed:
يَـٰٓأَيُّهَا ٱلنَّاسُ إِنَّا خَلَقْنَـٰكُم مِّن ذَكَرٍ وَأُنثَىٰ وَجَعَلْنَـٰكُمْ شُعُوبًا وَقَبَآئِلَ لِتَعَارَفُوا إِنَّ أَكْرَمَكُمْ عِندَ ٱللَّـهِ أَتْقَىٰكُمْ إِنَّ ٱللَّـهَ عَلِيمٌ خَبِيرٌ
O mankind! We created you from a single (pair) of a male and a female, and made you into nations and tribes, that you may recognise each other (not that you may despise each other). Verily the most honored of you in the sight of Allah is (he who is) the most righteous of you. And Allah has full knowledge and is well acquainted (with all things). (Surah Hujuraat)
(Tafseer Baghawi 4/195, Ibn Hishaam 4/43, Subulul Hudaa war Rashaad 5/248, Zaadul Ma‘aad 3/358-361)