Musnad Ahmad 5:793

Abu Sa'id narrated to us, Isra'il narrated to us, Abu Ishaq narrated to us, from 'Abd al-Rahman ibn Abi Layla, from 'Ali, may Allah be pleased with him, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Shall I not teach you words that, if you say them, you will be forgiven, even though you are already forgiven? There is no god but Allah, the Most High, the Magnificent. There is no god but He, the Forbearing, the Generous. Glory be to Allah, Lord of the Magnificent Throne. Praise be to Allah, Lord of the worlds."