Muslim :: Book 4 : Hadith 748 - The Book of Prayers (Kitab Al-Salat)

'Umar b. al-Khattab reported: The Messenger of Allah (may peace be upon him) said: When the Mu'adhdhin says:
Allah is the Greatest, Allah is the Greatest,
and one of you should make this response: Allah is the Greatest, Allah is the Greatest;
(and when the Mu'adhdhin) says: 
I testify that there is no god but Allah, one should respond: I testify that there is no god but Allah,
and when he says:
I testify that Muhammad is the Messenger of Allah, one should make a response: I testify that Muhammad is Allah's Messenger.
When he (the Mu'adhdhin) says: 
Come to prayer, one should make a response: There is no might and no power except with Allah.
When he (the Mu'adhdhin) says: 
Come to salvation, one should respond: There is no might and no power except with Allah, 
and when he (the Mu'adhdhin) says: 
Allah is the Greatest, Allah is the Greatest, then make a response: Allah is the Greatest, Allah is the Greatest. 
When he (the Mu'adhdhin) says: 
There is no god but Allah, 
and he who makes a re- sponse from the heart: There is no god but Allah, he will enter Paradise.