Musa bin Abi Uthman (Shu'bah said: he used to give the adhan on the tallest minaret in Kufah) said: Abu Yahya narrated to me while I was going round with him (that is, performing tawaf of the House of Allah); he said: I heard Abu Hurayrah (may Allah be pleased with him) say: I heard the Messenger of Allah (peace and blessings be upon him), I heard it from his (peace and blessings be upon him) own mouth: "The mu'adhdhin is forgiven to the extent of the reach of his voice, and every moist and dry thing testifies for him; and for the one who attends the prayer twenty-five good deeds are written, and (the sins) between them (the prayers) are expiated."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1861