Musnad Ahmad 435:97

Yunus narrated to us, Hammad narrated to us, from Asim ibn Bahdalah, from Abu Salih, from Mu'awiyah ibn Abi Sufyan: That the Messenger of Allah (peace and blessings of Allah be upon him), when he heard the mu'adhdhin say: "Allah is the Greatest, Allah is the Greatest," he would say the like of what he said. And when he said: "I bear witness that there is no god but Allah," he would say the like of what he said. And when he said: "I bear witness that Muhammad is the Messenger of Allah," he would say the like of what he said.