It is reported that Bilāl – Allāh be pleased with him – never used to call the adhān for anyone after the death of Allāh’s Messenger – Allāh’s peace and blessing be upon him. He wanted to go for al-jihād but Abū Bakr didn’t want him to go. So Bilāl said, “If you freed me [when I was a slave] for Allāh’s pleasure, then let me go.” So he went to Al-Shām. When ʿUmar arrived in Al-Jābiyah [during his Caliphate], the Muslims requested him to ask Bilāl to do the adhān for them, so ʿUmar asked him. He called the adhān one day, and never were as many people heard crying as on that day, as they remembered the Prophet – Allāh’s peace and blessing be upon him.
It is said that the adhān of the people of Al-Shām was taken from this adhān.
Al-Dhahabī, Siyar A’lām Al-Nubalā`, in his biography of Bilāl b. Abī Rabāḥ.