Musannaf Ibn Abi Shayba 21:319

Abu Usama narrated to us, from Husayn ibn Dhakwan, from Abdullah ibn Buraydah, from Bashir ibn Ka'b, from Shaddad ibn Yunus, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The master of seeking forgiveness is for one to say: 'O Allah, You are my Lord, and I am Your servant. There is no god but You. You created me, and I am Your servant. I have entered 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 Your favor upon me, and I acknowledge my sins, so forgive me, for indeed, no one forgives sins except You.'"