Musnad Abu Ya'la al-Mawsili 6:34

Abu Khaythama narrated to us, Yunus bin Muhammad narrated to us, Layth bin Sa'd narrated to us, from al-Hakam bin 'Abd Allah bin Qays—Abu Khaythama said: and some of them say Hakim bin 'Abd Allah—from 'Amir bin Sa'd, from his father Sa'd, from the Prophet (may Allah bless him and grant him peace), that he said: "Whoever says when he hears the caller to prayer: 'And I bear witness that there is no god but Allah alone, with no partner, and that Muhammad is His servant and His Messenger; I am pleased with Allah as a Lord, with Muhammad as a Messenger, and with Islam as a religion,' his sin will be forgiven for him."