Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) states: We were with the Prophet (peace and blessings be upon him) on a journey, and we heard a man saying "Allahu akbar, Allahu akbar, ashhadu an la ilaha illallah, ashhadu anna Muhammadan rasulullah (Allah is the Greatest, Allah is the Greatest, I bear witness that there is no god but Allah, I bear witness that Muhammad is the Messenger of Allah)" (that is, we heard him giving the adhan). He said, "He has come out of polytheism (shirk)."
Imam al-Bazzar recorded this narration, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1895