Shaddad ibn Aws (Allah be pleased with him) relates that the Prophet (Allah bless him and give him peace) said,
“The best of forgiveness (sayyid al-istighfar) is to say,
‘O Allah! You are my Lord. There is no god but. You created me and I am Your slave. I am on Your covenant and promise as much as I am able. I seek refuge in you from the wrongs I have done. I admit to You all the blessings You have bestowed upon me, and I admit to You my sins. So forgive me, for none forgives sins but You.’ ”
The Prophet (Allah bless him and give him peace) then said,
“Whoever says it during the day, with conviction, and then dies that day before night falls will be of the people of Paradise. And whoever says it at night, with conviction, and then dies before day break will be of the people of Paradise.” [Bukhari (6306), Tirmidhi (3393), Nasa’i (5522), and Ahmad (16662)]
More information on this dua:
http://qa.sunnipath.com/issue_view.asp?id=2869