Musnad Ahmad

It is narrated from Hazrat Mu'adh (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) heard a caller saying during one of his journeys: "Allahu Akbar, Allahu Akbar." The Prophet said, "This is upon the sound nature (fitrah)." Then the caller said, "Ashhadu an la ilaha illallah," so the Prophet (peace and blessings be upon him) said, "He has testified to the truth." When he said, "Ashhadu anna Muhammadur Rasulullah," the Prophet said, "He has come out of Hell. Go and see; either you will find him a shepherd who has separated himself or a captive." The people went and saw that he was a shepherd, and when the time for prayer had come, he had given the call to prayer (adhan).