Bilal — the slave whose voice was chosen
Source: Sahih al-Bukhari 30, 604, 1149, 1601, 3754; Sahih Muslim 2458; Quran 49:13; the torture and the adhan from the Kaaba from the Seerah of Ibn Ishaq
Bilal ibn Rabah was a slave in Makkah.
That sentence takes a second to read and holds a whole life. To be a slave in that society meant you were somebody's property — your time, your labour, your body, and in law your will as well. You had no tribe to protect you, and in Arabia protection meant a tribe.
A man in Makkah with no tribe could say anything he liked — but he paid for it alone.
And he said one thing.
He was of Abyssinian origin. His mother's name was Hamamah, and that detail matters later — because one day a man was insulted by his mother, and the answer given that day is preserved in Sahih al-Bukhari.
The first part below is from the Seerah; where a sound narration is used, the number is given.