The wording of Sayyidul Istighfar is:
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 عَلَيَّ وَأَبُوءُ لَكَ بِذَنْبِي فاغْفِرْ لِي فَإِنَّهُ لاَ يَغْفِرُ الذُّنُوبَ إِلاَّ 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 laka bi dhambi, faghfir li fa innahu la yaghfirudh dhunuba illa anta
Translation
O Allah, You are my Rabb. There is none worthy of worship besides you. You have created me and I am your servant [i.e I worship you]. I abide by your covenant and promise [of sincerely believing in you and worshipping you] to the best of my ability. I seek refuge in you from the evil I have done. I acknowledge your bounties upon me, and [at the same time] I [also] confess to/acknowledge my sins. Therefore, forgive me! Surely, the matter is such that only You forgive sins.
Full narration
Imam Bukhari (rahimahullah) has recorded on the authority of Sayyiduna Shaddad Ibn Aws (radiyallahu ‘anhu) that Rasulullah (sallallahu ‘alayhi wasallam) has said:
‘Sayyidul Istighfar (i.e the most comprehensive du’a to recite when seeking forgiveness) is to recite:
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 dhambi, faghfir li fa innahu la yaghfirudh dhunuba illa anta.
Whoever recites it in the morning, having utmost sincerity [hoping for forgiveness and reward], and then passes away on that day before entering the evening, will be among the people of Jannah [i.e he/she will enter Jannah]. And whoever recites it at night, having utmost sincerity, then passes away before entering the morning will be among the people of Jannah.’
(Sahih Bukhari, Hadith: 6306)
And Allah Ta’ala knows best.