Hadhrat Bilaal (Radhiallahu Anhu) was an Abyssinian slave who entered into the fold of Islam in the early days of Islam. He belonged to a woman of the Banu Jumah tribe. Umayyah bin Khalaf , the worst enemy of Islam, was her agent. He would make him lie down on the burning sand at midday and would place a heavy stone on his chest, so that he could not even move a limb. He would then say to him: “Renounce Islam or swelter and die.”
Even under these extreme afflictions, Hadhrat Bilal (Radhiallahu Anhu) would exclaim: “Ahad Ahad!” (There is only one Allah (who is my creator and worthy of worship)).
He used to be severely beaten and lashed at night. Despite the wounds and injuries sustained through the lashing, the following day the Mushrikeen would lay him down again on the burning ground in the scorching heat of Makkah Mukarramah so that he would renounce Islam. They would also drag Hadhrat Bilaal (Radhiallahu Anhu) through the two mountains of Makkah Mukarramah with a rope around his neck, torturing him in this manner. All this was done only in order that he forsake Islam or die a lingering, abject death in the process. The torturers (Abu Jahl, Umayyah and others) would take turns in punishing him. They would vie with one another to see who could inflict the most painful punishment on him. Despite the severe persecution and hardships, Hadhrat Bilaal (Radhiallahu Anhu) remained steadfast and would continuously say, “Ahad! Ahad!” (There is only one Allah (who is my creator and worthy of worship)).
One day Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu) passed by the Mushrikeen while they were meting out punishment upon Hadhrat Bilaal (Radhiallahu Anhu). Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu) could not bear it any longer to see Hadhrat Bilaal (Radhiallahu Anhu) in such pain and misery. He addressed Umayyah bin Khalaf (one of the leaders of Makkah) saying: “O Umayyah, do you not fear Allah Ta’aala for what you are doing to this weak slave? Until when will you continue torturing him and persecuting him so severely?”
Umayyah replied, “It is you people who have landed him into this misery (i.e. through introducing a new religion in our land and turning away from the religion of our forefathers). Hence, you may rescue him from this torment if you so wish.” Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu) while negotiating the price with Umayyah, offered to exchange the slave he had in place of Hadhrat Bilaal (Radhiallahu Anhu) as well as pay nine ooqiya of silver (three hundred and sixty Dirhams). Thus he said to Umayyah, “I have an Abyssinian slave who is stronger than him and a follower of your religion. If you so wish I am willing to exchange him for Hadhrat Bilaal (Radhiallahu Anhu).” Umayyah agreed to the exchange. Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu), upon purchasing Hadhrat Bilaal (Radhiallahu Anhu), immediately freed him for the sake of Allah Ta’aala.
Once the deal was done, Umayyah tauntingly mentioned to Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu): “By the qasm of Laat and Uzza, (since he is not worth anything in my sight) if you offered even one ooqiya to purchase him, I would have sold him to you. Hadhrat Abu Bakr Siddeeq (Radhiallahu Anhu) retorted: “I take a qasm on Allah Ta’aala, if I was forced to pay one hundred ooqiya (four thousand Dirhams) to purchase him (since he is worth so much in the sight of Allah Ta’aala), I would have accepted the offer and purchased him.” 
 أسد الغابة، حياة الصحابة