Bilal was at first a slave of Safwan bin Umayyah. During his slavery, he was tortured inhumanely because of his faith. He was made to lie down naked on the burning sand of the Arabian desert; a heavy stone was put on his chest which made breathing difficult for him. And as if it was not enough, some heavily built persons used to jump upon the stone, trying to crush him to death. Still the only sound heard from Bilal was “Ahad! Ahad! (One God! One God!).
Seeing such barbarism meted out to Bilal, the Prophet was very much grieved. Abu Bakr purchased and emancipated him.