Taught to Abū Bakr for inside the prayer
He asked the Prophet ﷺ for something to say in his ṣalāh. This was given.
قُلِ اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا ��َلاَ يَغْفِرُ الذُّنُوبَ إِلاَّ أَنْتَ، فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ، وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ
Allāhumma innī ẓalamtu nafsī ẓulman kathīrā, wa lā yaghfiru-dh-dhunūba illā ant, fa-ghfir lī maghfiratan min ʿindik, wa-rḥamnī, innaka anta-l-Ghafūru-r-Raḥīm.
“O Allah, I have wronged myself greatly, and none forgives sins but You. So forgive me with forgiveness from You, and have mercy on me. Indeed You are the Forgiving, the Merciful.”
He asks for forgiveness from Allah's side, not on the strength of anything he brought.
The full narration
Narrated Abu Bakr As-Siddiq:I asked Allah's Messenger (ﷺ) to teach me an invocation so that I may invoke Allah with it in my prayer. He told me to say, "Allahumma inni zalumtu nafsi zulman kathiran, Wala yaghfiru dh-dhunuba illa anta, fa ghfir li maghfiratan min `indika, wa r-hamni, innaka anta l-ghafuru r-rahim (O Allah! I have done great injustice to myself and none except You forgives sins, so bestow on me a forgiveness from You, and Have Mercy on me, You are the Forgiver, the Merciful)
Source
Ṣaḥīḥ al-Bukhārī · 834
- Agreed upon as authentic (al-Bukhārī / Muslim)