Sahih Bukhari: Hadith 894

I heard Muawiya bin Abi Sufyan (repeating the statements of the Adhan) while he was sitting on the pulpit. When the Mu'adh-dhin pronounced the Adhan saying, "Allahu-Akbar, Allahu Akbar", Muawiya said: "Allah Akbar, Allahu Akbar." And when the Mu'adh-dhin said, "Ash-hadu an la ilaha illal-lah (I testify that none has the right to be worshipped but Allah)", Muawiya said, "And (so do) I". When he said, "Ash-hadu anna Muhammadan Rasulullah" (I testify that Muhammad is Allah's Apostle), Muawiya said, "And (so do) I". When the Adhan was finished, Muawiya said, "O people, when the Mu'adh-dhin pronounced the Adhan I heard Allah's Messenger (ﷺ) on this very pulpit saying what you have just heard me saying".