Musnad Ahmad 1565

It was narrated from “Amir bin Sa"d bin Abi Waqqas, from his father Sa"d, that the Messenger of Allah (ﷺ) said: "Whoever says when he hears the mu"adhdhin; ‘And I bear witness that there is no God but Allah alone, with no partner or associate, and that Muhammad is His slave and His Messenger; I am content with Allah as my Lord, Muhammad as my Messenger and Islam as my religion, his sins will be forgiven him.