It is narrated from Sayyiduna Shaddad bin Aws (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to him: "Shall I not tell you of the best supplication for forgiveness? That you say: 'O Allah, You are my Lord; You created me, and I abide by Your covenant and promise as far as I am able. I seek refuge in You from the evil of what I have done. I acknowledge Your favour upon me, and I acknowledge my sins, for indeed none forgives sins except You.' No servant says it and then his death comes to him that same day before evening, or in his evening before morning, except that he will be among the people of Paradise, if Allah wills."
Hadith Referenceالمعجم الكبير للطبراني / باب الشين / 7158