Musannaf Ibn Abi Shayba 2:232

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, he said: Al-Hasan ibn al-Hakam narrated to me, he said: Ali ibn Abbad Abu Hubayrah narrated to me, from a sheikh, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The mu'adhdhin is forgiven to the extent of his voice's reach, and everything moist and dry testifies for him."