Mujammi' al-Ansari (may Allah have mercy on him) narrates that he heard Sayyiduna Abu Umamah bin Sahl bin Hunayf (may Allah be pleased with him): when he heard the muezzin say the takbir, he too said the takbir, and he bore witness just as the muezzin bore witness. Then he said: This is how Sayyiduna Mu'awiyah (may Allah be pleased with him) told us that he heard the Messenger of Allah (peace and blessings be upon him) saying what the muezzin said; and when the muezzin said: "I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah," he (peace and blessings be upon him) said: "And I bear witness too," and then he fell silent.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16769