When you carry guilt
The Prophet ﷺ called this the master of seeking forgiveness.
سَيِّدُ الاِسْتِغْفَارِ أَنْ تَقُولَ اللَّهُمَّ أَنْتَ رَبِّي، لاَ إِلَهَ إِلاَّ أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَىَّ وَأَبُوءُ لَكَ بِذَنْبِي، فَاغْفِرْ لِي، فَإِنَّهُ لاَ يَغْفِرُ الذُّنُوبَ إِلاَّ أَنْتَ
Allāhumma anta Rabbī, lā ilāha illā anta, khalaqtanī wa anā ʿabduk, wa anā ʿalā ʿahdika wa waʿdika ma-staṭaʿt. Aʿūdhu bika min sharri mā ṣanaʿt, abū'u laka bi-niʿmatika ʿalayya wa abū'u laka bi-dhanbī, fa-ghfir lī, fa-innahu lā yaghfiru-dh-dhunūba illā ant.
“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 seek refuge in You from the evil I have done. I acknowledge Your favour upon me, and I acknowledge my sin, so forgive me, for none forgives sins but You.”
The full narration
Narrated Shaddad bin Aus:The Prophet (ﷺ) said "The most superior way of asking for forgiveness from Allah is: '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 bini`matika `alaiya, wa abu'u laka bidhanbi faghfir lee fa innahu la yaghfiru adhdhunuba illa anta." The Prophet (ﷺ) added. "If somebody recites it during the day with firm faith in it, and dies on the same day before the evening, he will be from the people of Paradise; and if somebody recites it at night with firm faith in it, and dies before the morning, he will be from the people of Paradise
Source
Ṣaḥīḥ al-Bukhārī · 6306
- Agreed upon as authentic (al-Bukhārī / Muslim)