Umayyah replied, “You are responsible for corrupting him, so now you are obliged to release him. Hazrat Abu Bakr (radhiyallahu ‘anhu) replied, “Very well! I have a slave who is extremely strong, and is firm and steadfast on your faith. Take him in exchange of Hazrat Bilaal (radhiyallahu ‘anhu), and give Hazrat Bilaal (radhiyallahu ‘anhu) to me. Umayyah accepted this offer. Hazrat Abu Bakr (radhiyallahu ‘anhu) then took Hazrat Bilaal (radhiyallahu ‘anhu) and set him free.

The brutal torture of the disbelievers had left scars and marks on the back of Hazrat Bilaal (radhiyallahu ‘anhu), the best of all inviters to Allah Ta‘ala (and mu’azzins).these scars were clearly visible whenever his back would be exposed.

(Extracted from Seeratul Mustafa 1/224-225)