Sunan An-Nasai, Chapter: The Book of the Adhan (The Call to Prayer)

that the Messenger of Allah (saw) said: "Whoever says, when he hears the Muadhdhin: 'Ashhadu an la ilaha illallah wahdahu la sharika lahu wa anna Muhammad (SAW) an abduhu wa Rasuluhu, raditu Billahi Rabban, was bil-Islami dinan wa bi Muhammad (SAW) in Rasula (I bear witness that there is none worthy of worship except Allah alone, with no partner or associate, and that Muhammad (SAW) is His slave and Messenger; I am content with Allah as my Lord, Islam as my religion and Muhammad (SAW) as my Messenger),' his sins will be forgiven." (Sahih)