It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The one who calls the adhan is forgiven as far as his voice reaches, and his reward is equal to the reward of those who pray with him."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7911