It is narrated that Sayyiduna Bilal (may Allah be pleased with him) came to Sayyiduna Abu Bakr (may Allah be pleased with him) and said: O Caliph of the Messenger of Allah (peace and blessings be upon him)! I heard the Messenger of Allah (peace and blessings be upon him) say: "The best deed of a believer is jihad in the path of Allah," and I wish to devote myself to the path of Allah until I die. Abu Bakr (may Allah be pleased with him) said: I adjure you by Allah, O Bilal, and by my sanctity and my right (over you): my age has advanced, my strength has weakened and my death has drawn near. So Bilal (may Allah be pleased with him) stayed with him. Then when Abu Bakr (may Allah be pleased with him) passed away, Sayyiduna Umar (may Allah be pleased with him) came and said the same thing to him that Abu Bakr (may Allah be pleased with him) had said, but Bilal (may Allah be pleased with him) refused. Umar (may Allah be pleased with him) said: Then to whom will you go, O Bilal? Bilal (may Allah be pleased with him) said: To Sa'd, for he gave the call to prayer (adhan) at Quba in the time of the Messenger of Allah (peace and blessings be upon him). So Umar (may Allah be pleased with him) entrusted (the responsibility of) the adhan to Sa'd and his descendants.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1009