Once in the morning and once in the evening. It is the “master 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 بِذَنْبِي فَاغْفِرْ لِي، فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ
Allāhumma anta rabbī lā ilāha illā ant, khalaqtanī wa anā ʿabduk, wa anā ʿalā ʿahdika wa waʿdika mā-staṭaʿt, aʿūdhu bika min sharri mā ṣanaʿt, abū’u laka bi-niʿmatika ʿalayya, wa abū’u bi-dhanbī fa-ghfir lī, fa-innahu lā yaghfiru-dh-dhunūba illā ant
O Allah, You are my Lord, none has the right to be worshipped but You. You created me and I am Your servant. I keep Your covenant and promise as much as I can. 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.
Sahih al-Bukhari 6306Its merit “Whoever says it during the day with conviction and dies before evening is among the people of Paradise; whoever says it at night with conviction and dies before morning is among the people of Paradise.”