(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