It was narrated that Mujammi' bin Yahya Al-Ansari said:
"I was sitting with Abu Umamah bin Sahl bin Hunaif when the Mu'adhdhin 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 ialaha ill-Allah (I bear witness that there is none worthy of worship except Allah),' and he also sent the testimony twice. Then he said: 'Ashhadu anna Muhammadan Rasul-Allah (I bear witness that Muhammad is the Messenger of Allah),' and he (also) sent the testimony twice. Then he said: 'This is what Mu'awiyah bin Abi Sufyan told me, narrating from statement of the Messenger of Allah.'"
English reference: Vol. 1, Book 7, Hadith 676
Arabic reference: Book 7, Hadith 683