Abu Mahdurah reported: The Messenger of Allah (peace be upon him) himself taught me the call to prayer (adhan). He asked me to pronounce: Allah is most great. Allah is most great. Allah is most great. Allah is most great: I testify that there is no god but Allah. I testify that there is no god but Allah; I testify that Muhammad is the Messenger of Allah (peace be upon him), I testify that Muhammad is Messenger of Allah. Then repeat and raise your voice; I testify that there is no god but Allah, I testify that there is no god but Allah ; I testify that Muhammad is the Messenger of Allah (peace be upon him); I testify that Muhammad is the Messenger of Allah(peace be upon him); come to prayer, come to prayer; come to salvation, come to salvation; Allah is most great. Allah is most great; there is no god but Allah.
Sunan Abi Dawud, Book 2, Hadith 113