Sunan An-Nasai, Chapter: The Book of the Adhan (The Call to Prayer)

"I was sitting with Abu Umamah bin Sahl bin Hunaif when the Muadhdhin called the Adhan. He said: 'Allahu Akbar, Allahu Akbar (Allah is the Greatest, Allah is the Greatest),' and he (also) pronounced the Takbir twice. Then he said: 'Ashhadu an la ilaha illallah (I bear witness that there is none worthy of worship except Allah),' and he also sent the testimony twice. Then he said: 'Ashhadu anna Muhammad (SAW)an Rasulullah (I bear witness that Muhammad (SAW) is the Messenger of Allah),' and he (also) sent the testimony twice. Then he said: 'This is what Muawiyah bin Abi Sufyan told me, narrating from statement of the Messenger of Allah (saw).'" (Sahih)