Jami` at-Tirmidhi 210

Sa'd bin Abi Waqqas narrated that :Allah's Messenger said: "Whoever says, when he hears the Mu'adh-dhin: (Wa Ana Ashadu An La Ilaha Illallah, Wahdahu La Sharika Lahu, Wa Anna Muhammadan Abduhu Wa Rasuluhu, Radittu Billahi Rabban Wa Bil-Islam Dinan, Wa Bi Muhammadin Rasulan) 'I too testify that none has the right to be worshiped but Allah, Alone without partners, and that Muhammad is His slave and Messenger, I am pleased with Allah as my Lord, with Islam as my religion and Muhammad as a Messenger.' - Allah will pardon his sins for him