The Dua the Prophet Taught Abu Bakr
بسم الله الرحمن الرحيم
Abu Bakr reported that he said to Allah’s Messenger (ﷺ):
قُلِ: اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَبِيرًا وَلاَ يَغْفِرُ الذُّنُوبَ إِلاَّ أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ ” .
Teach me a supplication which I should recite in my prayer. After his request the Prophet(ﷺ) said: Recite:” O Allah, I have done a great wrong to my soul and there is none to forgive the sins, except You, ” Grant me a pardon from Yourself and have mercy on me; as You are the Most Forgiving and the Most Compassionate.”
Benefits for this subject
Imam Ibn Al-Jazwi (597H)
(اللهم: Allahumma) means Oh Allah its’ similar to saying “ Ya Allah
I have done great wrong to myself… The word “ الظلم: Thulm ” means to place something in its improper place. Others have said it means to spend or share something that you don’t own. Also, the meaning of doing wrong has been explained to mean to regularly sin, so to wrong yourself is when you follow your desires doing things that deserve a punishment. This could refer to acts which decrease the rewards of good deeds or diminish their virtue.
“فاغفرلي “ forgiveness is the covering of a sin and pardoning of it as if it never happened.
Grant me pardon from Yourself… What does this mean? Is there anyone who can forgive sins other than Allah? The answer to this question is you are saying: award me with forgiveness from Your bounty, although I don’t deserve it based on deeds.
This is among the best supplications. The servant acknowledges he’s wronged himself and recognizes he’s sinned. Sins block blessings and identifying your sins is a way to remove the barrier between them and your blessings.
It’s recommended to make this dua at the end of the prayer before the Tasleem. The person can make dua with anything found in the Quran and authentically reported on the Prophet [ﷺ], and should avoid supplicating with any other wording, including the speech of people. 
Translated by ,
Abu Aaliyah Abdullah ibn Dwight Battle
Doha, Qatar, 1439©
 Kashful Mushkil / vol 1 pg 12-13