The Prophet [SAW] said: "The best of prayers for forgiveness is for a person to say: 'Allahumma, anta rabbi, la ilaha illa anta, khalaqtani wa ana 'abduka, wa ana 'ala 'ahdika wa wa'dika mastata'tu, a'udhu bika min sharri ma sana'tu, abuw'u laka bidhanbi, wa abuw'u laka bini'matika 'alayya faghfirli, fa innahu la yaghfirudh-dhunuba illa anta (O Allah, You are my Lord, there is no god but You. You have created me and I am Your slave and I am keeping my promise and covenant to You as much as I can. I seek refuge with You from the evil of what I do. I acknowledge Your blessing and I acknowledge my sin, so forgive me, for there is none who can forgive sin except You.)' If he says this in the morning, believing in it firmly, and dies on that day before evening comes, he will enter Paradise, and if he says it in the evening, believing firmly in it, and dies before morning comes, he will enter Paradise." Al-Walid bin Tha'labah contradicted him. (Using translation from Nasāʾī 5522)
عَنِ النَّبِيِّ ﷺ قَالَ سَيِّدُ الِاسْتِغْفَارِ أَنْ يَقُولَ الْعَبْدُ اللهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَبُوءُ لَكَ بِالنِّعْمَةِ وَأَبُوءُ لَكَ بِذَنْبِي فَاغْفِرْ لِي إِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ قَالَ إِنْ قَالَهَا بَعْدَمَا يُصْبِحُ مُوقِنًا بِهَا ثُمَّ مَاتَ كَانَ مِنْ أَهْلِ الْجَنَّةِ وَإِنْ قَالَهَا بَعْدَمَا يُمْسِي مُوقِنًا بِهَا ثُمَّ مَاتَ كَانَ مِنْ أَهْلِ الْجَنَّةِ