Musnad Ahmad 27:1742

Mu'awiyah narrated to us, Za'idah narrated to us, from al-A'mash, from a man, from Ibn 'Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "Allah forgives the mu'adhdhin (the one who calls the adhan) to the extent of his voice, and everything moist and dry that hears his voice seeks forgiveness for him."