Narrated Sa‘d b. Abu Waqqas reported God’s Messenger as saying:
If anyone says when he hears the mu’adhdhin, “I testify that there is no god but God alone who has no partner and that Muhammad is His servant and Messenger; I am satisfied with God as Lord, with Muhammad as Messenger, and with Islam as religion,” his sins will be forgiven him.
Anas said that the Prophet used to attack the enemy at dawn, and he would sometimes hear the adhan, so if he heard an adhan he stopped, but otherwise he attacked. Once on hearing a man say, “God is most great, God is most great,” God’s Messenger said, “You follow Islam.” Then hearing him say, “There is no god but God,” he said, “You have come forth from hell.” They looked at him and found that he was a goatherd.
‘Abdallah b. Mughaffal reported God’s Messenger as saying, “Between every pair of adhans there is a prayer; between every pair of adhans there is a prayer.” Then he said on repeating it a third time, “for him who wishes.”
Narrated Jabir reported God’s Messenger as saying:
If anyone says when he hears the summons, “O God, Lord of this perfect call and of the prayer which is established for all time, grant Muhammad the wasila and excellency, and raise him up in a praiseworthy position (Al-Qur’an; 17:79) which Thou hast promised,” he will be assured of my intercession.