It was narrated from Sa'd bin Abi Waqqas that the Messenger of Allah (ﷺ) said:
"Whoever says, when he hears the Mu'adhdhin: 'Ashhadu an la ilaha illallah wahdahu la sharika lahu wa anna Muhammadan 'abduhu wa Rasuluhu, raditu Billahi Rabban, wa bil-Islami dinan was bi Muhammadin Rasula (I bear witness that there is none worthy of worship except Allah alone, with no partner or associate, and that Muhammad is the His slave and Messenger; I am content with Allah as my Lord, Islam as my religion and Muhammad as my Messenger),' his sins will be forgiven."
English reference: Vol. 1, Book 7, Hadith 680
Arabic reference: Book 7, Hadith 687