Musnad Ahmad 27:1741

Abu al-Jawwab narrated to us, Ammar ibn Ruzayq narrated to us, from al-A’mash, from Mujahid, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah forgives the mu'adhdhin (the one who calls the adhan) to the extent of his voice's reach, and everything, wet or dry, that hears his voice bears witness for him."