Hazrat Bilaal (radhiyallahu ‘anhu) was an Abyssinian by lineage, and was the slave of Umayyah bin Khalaf.
At midday, when the heat was at its fiercest and the boulders of the desert would become extremely hot, Umayyah would command his slaves to lay Hazrat Bilaal (radhiyallahu ‘anhu) down onto the burning stones of the desert. He would then place a heavy boulder onto his chest to prevent him from moving and would say, “You will die like this. If you wish to live, then reject Muhammed (sallallahu ‘alaihi wasallam) and worship Laat and ‘Uzza.” However, nothing but the words ‘Ahad! Ahad!’ (Allah is One! Allah is One!) would emit from his mouth.
Umayyah would sometimes cover him in cowhide or make him wear a suit of armour and force him to sit in the scorching sun. But even in this state of torture, the words ‘Ahad! Ahad!’ would emanate from his tongue.
When Umayyah saw the determination of Hazrat Bilaal (radhiyallahu ‘anhu) and realized that he was not deterred in the least, he tied a rope around his neck and made him over to some young boys who continued dragging him round the city. However, he still persisted in uttering the words ‘Ahad! Ahad!’.
(Extracted from Seeratul Mustafa 1/224)