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
According to the Seerah, when he accepted Islam his master Umayyah ibn Khalaf would take him out in the fierce midday sun, lay him on the burning sand, have a great stone put on his chest, and tell him to deny Muhammad and speak of Lat and Uzza.
And what he said was one word, over and over:
"Ahad. Ahad." — One. One.
That detail does not come in this form in Sahih al-Bukhari or Sahih Muslim; it is from Ibn Ishaq's Seerah and books of that rank. It is well known and is in the reliable works of Seerah, but it does not carry the standing of what comes in the parts after it.
One thing here is worth noticing, and it is about the word. He was being told to deny, and he was not denying — but what he said was not an argument either. It was one word: one.
A man with no power, no tribe, no property — he had one word, and he did not let go of it.