The best supplication for asking Allah for forgiveness by MyIslaam
 

Shadad ibn Aws Allah be pleased with him narrates that the messenger of Allah peace be upon him said, ‘The best supplication for asking Allah for forgiveness is:

Oh Allah, You are my Lord, there is none worthy of worship besides You, You created me and I am your slave, I abide by your covenant and the promise I made to you as much as I can, I seek refuge in you from that which I have done, I acknowledge all the favours you have done to me and all the sins I have committed, so forgive me as no one can forgive sins except you.

Whoever says these words in the day with full conviction in them and then dies on the same day before he sees evening then he will be from the people of Jannah, and whoever says them at night having full conviction in them and then dies before he sees morning then he will be from the people of Jannah.’

عن شَدَّاد بْن أَوْسٍ رَضِيَ اللَّهُ عَنْهُ
عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، سَيِّدُ الِاسْتِغْفَارِ أَنْ تَقُولَ

اللَّهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ وَأَبُوءُ لَكَ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ

قَالَ وَمَنْ قَالَهَا مِنْ النَّهَارِ مُوقِنًا بِهَا فَمَاتَ مِنْ يَوْمِهِ قَبْلَ أَنْ يُمْسِيَ فَهُوَ مِنْ أَهْلِ الْجَنَّةِ وَمَنْ قَالَهَا مِنْ اللَّيْلِ وَهُوَ مُوقِنٌ بِهَا فَمَاتَ قَبْلَ أَنْ يُصْبِحَ فَهُوَ مِنْ أَهْلِ الْجَنَّةِ

(رواه البخاري: بَاب أَفْضَلِ الِاسْتِغْفَارِ)