Musnad Ahmad 435:95

Yahya ibn Ishaq narrated to us, Hammad ibn Salamah narrated to us, from Asim ibn Bahdalah, from Abu Salih, from Mu'awiyah ibn Abi Sufyan, who said: I heard the Prophet, peace and blessings of Allah be upon him, say: "When the mu'adhdhin (caller to prayer) gives the adhan, say the like of what he says."