Sunan Ibn Majah, Chapter: The Chapters on the Adhan and the Sunnah Regarding it

that the Messenger of Allah (saw) said: "Whoever says, when he hears the Mu'adh-dhin, 'Wa ana Ash-hadu an la ilaha illallah wahdahu la sharika lahu, wa ash-hadu anna Muhammad (SAW)an 'abduhu wa rasuluhu, radaytu Billahi rabban wa bil-Islami dinan wa bi Muhammad (SAW)in nabiyyan (And I bear witness that none has the right to be worshipped but Allah alone, with no partner, and I bear witness that Muhammad (SAW) is His slave and Messenger, and I am content with Allah as my Lord, Islam as my religion and Muhammad (SAW) as my Prophet),' his sins will be forgiven to him." (Sahih)