It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) had two appointed muezzins: Bilal and Ibn Umm Maktum (may Allah be pleased with them both), and the latter was blind. The Messenger of Allah (peace and blessings be upon him) said: "Bilal (may Allah be pleased with him) gives the adhan while it is still night, so eat and drink until Ibn Umm Maktum gives the adhan." And there was only as much time between them as it took for one to come down (from the minaret) and the other to go up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2014
(a) It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) had three appointed muezzins: Bilal, Abu Mahdhurah and Ibn Umm Maktum (may Allah be pleased with them). (b) Abu Bakr (may Allah have mercy on him) said: Both reports are sahih (authentic). Those who said there were two muezzins meant the two muezzins of Madinah, and those who said three meant to include Sayyiduna Abu Mahdhurah (may Allah be pleased with him) as well, who used to give the adhan in Makkah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2016
Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said that the third adhan was that of Friday; Sayyiduna Uthman (may Allah be pleased with him) ordered it when the population of Madinah grew, and (before that) the adhan on Friday was given when the imam sat on the minbar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2017