Musnad Ahmad 30:493

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Mansur, from 'Abbad ibn Anis, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him): "Indeed, the Mu'adhdhin is forgiven to the extent of his voice, and everything—wet or dry—that hears him testifies for him. And for the one who witnesses it [the prayer] with him, there are twenty-five degrees."