It is reported that Bilâl – Allâh be pleased with him – never used to call the adhân for anyone after the death of Allâh’s Messenger – Allâh’s peace and blessing be upon him. He wanted to go for al-jihâd but Abû Bakr didn’t want him to go. So Bilâl said, “If you freed me [when I was a slave] for Allâh’s pleasure, then let me go.” So he went to Al-Shâm. When ‘Umar arrived in Al-Jâbiyah [during his Caliphate], the Muslims requested him to ask Bilâl to do the adhân for them, so ‘Umar asked him. He called the adhân one day, and never were as many people heard crying as on that day, as they remembered the Prophet – Allâh’s peace and blessing be upon him.
It is said that the adhân of the people of Al-Shâm was taken from this adhân.
Al-Dhahabî, Siyar A’lâm Al-Nubalâ`, in his biography of Bilâl b. Abî Rabâh.