‘Abdullah ibn ‘Amr narrates that Abu Bakr al-Siddeeq (May Allah be pleased with him) said to the prophet (May Allah’s peace and blessings be upon him), “O messenger of Allah, teach me an invocation with which I may invoke (Allah) in my Salah.” He said, “Say:

O Allah, verily I have oppressed myself very much, and none forgives sins except you, so grant me forgiveness, forgiveness from you. Verily you are the most forgiving, most merciful.

(Narrated by al-Bukhari in the book of the oneness of Allah, Hadith 6839)

عَبْدَ اللَّهِ بْنَ عَمْرٍو أَنَّ أَبَا بَكْرٍ الصِّدِّيقَ رَضِيَ اللَّهُ عَنْهُ قَالَ لِلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَا رَسُولَ اللَّهِ عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلَاتِي قَالَ قُلْ :

اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مِنْ عِنْدِكَ مَغْفِرَةً إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ

 (رواه البخاري في صحيحه في كتاب التوحيد، حديث ٦٨٣٩)

NOTE: In Salah, it is Sunah to supplicate with words that resemble the Quran and the Sunah before the final Salam. However it is not nessasarry that the actual words originate from the Quran or Sunah although many supplications for this occasion can be found in many narrations.  It should be noted that the above narration also has variations.