Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) once mentioned:
It is mentioned in the Hadith that Rasulullah (sallallahu ‘alaihi wasallam) taught Hazrat Abu Bakr (radhiyallahu ‘anhu) the following du‘aa and instructed him to recite it before making salaam at the end of his salaah:
اَللَّهُمَّ إِنِّيْ ظَلَمْتُ نَفْسِيْ ظُلْمًا كَثِيْرًا وَلَا يَغْفِرُ الذُّنُوْبَ إِلَّا أَنْتَ فَاغْفِرْ لِيْ مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِيْ إِنَّكَ أَنْتَ الْغَفُوْرُ الرَّحِيْم
O Allah! I have oppressed myself excessively (through committing sins), and only You forgive sins, so forgive me through Your grace and have mercy on me, for indeed You alone are all-forgiving and all-merciful.
Let us reflect over the fact that Rasulullah (sallallahu ‘alaihi wasallam) taught this du‘aa to none other than Hazrat Abu Bakr (radhiyallahu ‘anhu). Hazrat Abu Bakr (radhiyallahu ‘anhu) was the greatest of this ummah and his salaah was the most perfect. Therefore, Rasulullah (sallallahu ‘alaihi wasallam) appointed him as the Imaam to lead the Sahaabah (radhiyallahu ‘anhum) in salaah during his final illness. However, despite the esteemed position of Hazrat Abu Bakr (radhiyallahu ‘anhu), Rasulullah (sallallahu ‘alaihi wasallam) taught him to recite this du‘aa at the end of his salaah, to acknowledge and confess that he had not fulfilled the salaah in accordance to the majesty of Allah Ta‘ala, and to beg Allah Ta‘ala, out of His grace, to forgive him for his shortcomings. (Malfoozaat Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) pg. 20-21)