Musnad Ahmad 658:39

'Ali ibn 'Abdullah narrated to us, Mu'adh narrated to us, my father narrated to me, from Qatadah, from Abu Ishaq al-Kufi, from al-Bara' ibn 'Azib, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "Indeed, Allah and His angels send blessings upon the front row. And the Mu'adhdhin (caller to prayer) is forgiven for the extent of his voice, and he is testified to by whatever hears him, whether moist or dry, and he has the like of the reward of whoever prays with him."