Musannaf Ibn Abi Shayba 21:129

Yahya ibn Ishaq narrated to us, Layth ibn Sa’d narrated to us, from al-Hakim ibn ‘Abd Allah ibn Qays, from ‘Amir ibn Sa’d, from his father Sa’d, that he said: Whoever says, when the mu’adhdhin says, "I bear witness that there is no god but Allah," [and he says]: "I am pleased with Allah as a Lord, with Islam as a religion, and with Muhammad as a Prophet," his sins will be forgiven. A man said to him, "O Sa’d, [what about] what preceded of his sins and what was delayed?" He said, "No, this is how I heard the Messenger of Allah, may Allah bless him and grant him peace, saying it."