Musannaf Ibn Abi Shayba 21:320

Zayd ibn al-Hubab narrated to us, he said: Kathir ibn Zayd narrated to me, he said: al-Mughirah ibn Sa'id ibn Nawfal narrated to me, from Shaddad ibn Aws, that the Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Shall I not guide you to the master of seeking forgiveness? That you say: 'O Allah, You are my God, there is no god but You. You created me and I am Your servant, and I am upon Your covenant and Your promise as much as I am able. I seek refuge in You from the evil of what I have done. I acknowledge to You Your favor upon me, and I acknowledge to You my sins, so forgive me, for indeed, no one forgives sins except You.' There is no servant who says it, and then his decree comes to him in his day before he reaches the evening, or in his evening before he reaches the morning, but that he will be among the people of Paradise."