Sayyid al-Istighfar — the master supplication of seeking forgiveness
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
Transliteration: Allahumma anta Rabbi la ilaha illa anta, khalaqtani wa ana abduk, wa ana ala ahdika wa waʼdika ma stataʼtu, audhu bika min sharri ma sanaʼtu, abuʼu laka bi-niʼmatika alayya wa abuʼu laka bi-dhanbi faghfir li, fa-innahu la yaghfiru-dh-dhunuba illa ant.
Translation: O Allah, You are my Lord. There is no god but You. You created me and I am Your servant. I keep Your covenant and Your pledge as best I can. I seek refuge in You from the evil I have done. I acknowledge Your blessings upon me and I acknowledge my sin, so forgive me — for none can forgive sins but You.