Sahih Muslim Book 4, Hadith Number 748.

‘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 response from the heart: There is no god but Allah, he will enter Paradise.