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: "Shall I not tell you of the best supplication for forgiveness (sayyid al-istighfar)?" The Companions said: Certainly, O Messenger of Allah! He (peace and blessings be upon him) said: "'O Allah, there is no true deity except You. You created me and I am Your servant, 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, and I acknowledge my sins, so forgive me, for indeed none forgives sins except You.' Whoever says it in the evening and dies that night, Paradise becomes due for him; and whoever says it in the morning and death comes to him that day, Paradise becomes due for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الشين / 7156