The adhan was assigned to the descendants of Abu Mahdhurah

Ibn Abi Mahdhurah narrates from his father, who said: The Messenger of Allah (peace and blessings be upon him) assigned the "Siqayah" (providing water to the pilgrims) to Banu Abd al-Muttalib and the "Hijabah" (custodianship and keeping of the keys of the Ka'bah) to Banu Abd al-Dar, and he assigned the office of the call to prayer (adhan) to us and our freed slaves (mawali).

Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) ordered Sayyiduna Abu Mahdhurah (may Allah be pleased with him) to say the phrases of the call to prayer (adhan) twice each and the phrases of the iqamah once each.

Abd al-Aziz bin Abd al-Malik bin Abi Mahdhurah narrates that Abdullah bin Muhayriz informed him; he was an orphan under the care and upbringing of Sayyiduna Abu Mahdhurah bin Mi'yar (may Allah be pleased with him), until he equipped him and sent him off to Syria.

It is narrated from Ibn Abi Mulaykah that the muezzin of Sayyiduna Mu'awiyah (may Allah be pleased with him) gave the call to prayer (when in Makkah), so Sayyiduna Abu Mahdhurah (may Allah be pleased with him) picked him up and threw him into the well of Zamzam.
Next comes the mention of the virtues of Sayyiduna Abu Usayd al-Sa'idi (may Allah be pleased with him).