Hazrat Sa‘eed bin Musayyib (rahimahullah) narrates that Hazrat Abu Bakr (radhiyallahu ‘anhu) had once sat on the mimbar on the occasion of jumu‘ah when Hazrat Bilaal (radhiyallahu ‘anhu) approached him and asked, “Did you grant me freedom for the sake of Allah Ta‘ala or for your own sake?” Hazrat Abu Bakr (radhiyallahu ‘anhu) instantly replied, “For the sake of Allah Ta‘ala.” Hazrat Bilaal (radhiyallahu ‘anhu) responded, “In that case, allow me to leave Madinah Munawwarah so that I may strive in jihad.” Hazrat Abu Bakr (radhiyallahu ‘anhu) permitted Hazrat Bilaal (radhiyallahu ‘anhu) to leave after which he went to Shaam which is where he eventually passed away.

(Siyar A‘laam min Nubalaa 3/221)

The Second View:

The second view is that after the demise of Rasulullah (sallallahu ‘alaihi wasallam), during the khilaafat of Hazrat Abu Bakr (radhiyallahu ‘anhu), Hazrat Bilaal (radhiyallahu ‘anhu) remained the mu’azzin of Madinah Munawwarah. After the demise of Hazrat Abu Bakr (radhiyallahu ‘anhu), he obtained permission from Hazrat ‘Umar (radhiyallahu ‘anhu) and departed for Syria.

This is the preferred view of ‘Allaamah ibnul Atheer (rahimahullah) (Usdul Ghaabah 1/237) and Haafiz ibnul Hajar ‘Asqalaani (rahimahullah) (Fat-hul Baari 7/125).

When Rasulullah (sallallahu ‘alaihi wasallam) passed away, Hazrat Bilaal (radhiyallahu ‘anhu) approached Hazrat Abu Bakr (radhiyallahu ‘anhu) seeking permission to leave Madinah Munawwarah so that he may strive in Jihaad. When he came before Hazrat Abu Bakr (radhiyallahu ‘anhu), he said to him, “O Khalifah of Rasulullah (sallallahu ‘alaihi wasallam)! I heard Rasulullah (sallallahu ‘alaihi wasallam) mentioning that the most virtuous action of a believer is waging Jihaad in the path of Allah Ta‘ala.” Hazrat Abu Bakr (radhiyallahu ‘anhu) replied, “So what do you wish, O Bilaal (radhiyallahu ‘anhu)?” Hazrat Bilaal (radhiyallahu ‘anhu) replied, “I wish to wage Jihaad on the borders of the Islamic lands until I pass away.” On hearing his request, Hazrat Abu Bakr (radhiyallahu ‘anhu) pleaded with him saying, “O Bilaal (radhiyallahu ‘anhu)! I ask you (to remain in Madinah Munawwarah) for the sake of Allah Ta‘ala and for the right which I have over you as I have now aged, I have become weak and my death is close approaching.” Out of respect for the request of Hazrat Abu Bakr (radhiyallahu ‘anhu), Hazrat Bilaal (radhiyallahu ‘anhu) remained in Madinah Munawwarah and continued to call out the azaan.

When Hazrat Abu Bakr (radhiyallahu ‘anhu) passed away, Hazrat Bilaal (radhiyallahu ‘anhu) approached Hazrat ‘Umar (radhiyallahu ‘anhu) and once again requested to leave Madinah Munawwarah so that he may strive in Jihaad. Hazrat ‘Umar (radhiyallahu ‘anhu) eventually permitted him to leave and asked him who should be appointed as the mu’azzin of Madinah Munawwarah in his place. Hazrat Bilaal (radhiyallahu ‘anhu) replied, “Appoint Hazrat Sa‘d (radhiyallahu ‘anhu) as he had called out the azaan for Rasulullah (sallallahu ‘alaihi wasallam).” Hazrat ‘Umar (radhiyallahu ‘anhu) thus appointed Hazrat Sa‘d (radhiyallahu ‘anhu) and his progeny to call out the azaan in Madinah Munawwarah.

(Siyar A‘laam min Nubalaa 3/221, ibn Sa‘d 3/178)