Sayyiduna Abu Umamah bin Sahl bin Hunayf said: I heard Sayyiduna Mu'awiyah (may Allah be pleased with him): when the muezzin said the takbir twice, he said the takbir twice, and when he bore witness twice, he bore witness twice. Then he turned to me and said: "This is how I heard the Messenger of Allah (peace and blessings be upon him) say it at the time of the adhan."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16768