He heard it directly from the mouth of the Messenger of Allah ﷺ saying, "The muezzin is forgiven to the extent of his voice's reach, and every moist and dry thing bears witness for him."
أخبرنا إسمعيل بن مسعود ومحمد بن عبد الأعلى قالا حدثنا يزيد يعني ابن زريع قال حدثنا شعبة عن موسى بن أبي عثمان عن أبي يحيى
سمعه من فم رسول الله ﷺ يقول المؤذن يغفر له بمد صوته ويشهد له كل رطب ويابس
[Its chain is hasan] Abu Dawud narrated it, Prayer, Chapter on Raising the Voice for the Adhan, Hadith 515, and Ibn Majah, Adhan, Chapter on the Merit of the Adhan and the Reward of Muezzins, Hadith 724, from the hadith of Shubah, with this wording, and it is also in Al-Kubra, Hadith 1609, and Ibn Hibban graded it sahih (Mawarid), Hadith 292.