Sahih Muslim, Chapter: Prayers (Kitab Al-Salat)

The Messenger of Allah (may peace be upon him) used to attack the enemy when it was dawn. He would listen to the Adhan; so if he heard an Adhan, he stopped, otherwise made an attack. Once on hearing a man say: Allah is the Greatest, Allah is the Greatest, the Messenger of Allah (may peace be upon him) remarked: He is following al-’Fitra (al-’Islam). Then hearing him say: I testify that there is no god but Allah. there is no god but Allah, the Messenger of Allah (may peace be upon him) said: You have come out of the Fire (of Hell). They looked at him and found that he was a goatherd.