The Master Supplication for Forgiveness
اللَّهُمَّ أَنْتَ رَبِّي لَا إِلَٰهَ إِلَّا أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلَىٰ عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ، وَأَبُوءُ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ
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, abu'u laka bi-ni'matika 'alayya, wa abu'u bi-dhanbi faghfir li fa-innahu la yaghfirudh-dhunuba illa anta.
O Allah, You are my Lord. There is no god but You. You created me and I am Your servant, and I hold to Your covenant and Your promise as much as I am able. I take refuge with You from the evil of what I have done. I acknowledge Your favour upon me and I acknowledge my sin, so forgive me, for none forgives sins except You.
What it means
This is called the master of asking forgiveness because it contains everything a return to Allah needs. You confess who He is, who you are, that you are trying and falling short, that His kindness is real and your sin is real, and that only He can settle it. The honesty of the line about keeping the covenant as much as I am able is what makes it usable by an ordinary struggling person.
When and how
Say it once in the morning and once in the evening. Do not rush it; the reward in the hadith is tied to saying it with conviction.
Its virtue
The Prophet ﷺ said that whoever says it during the day with certainty in it and dies that day before evening is among the people of Paradise, and whoever says it at night with certainty and dies before morning is among the people of Paradise, reported in Sahih al-Bukhari 6306.
Source
Al-Bukhari 6306sahih