Musnad Ahmad 30:2205

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Musa ibn Abi 'Uthman, he said: I heard Abu 'Uthman, he said: I heard Abu Hurairah saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The Mu'adhdhin is forgiven to the extent of his voice, and everything wet and dry bears witness for him. And the one who attends the prayer is written for him twenty-five good deeds, and what is between them is expiated for him."