عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو عَنْ أَبِي بَكْرٍ الصِّدِّيقِ رَضِيَ اللَّهُ عَنْهُ أَنَّهُ قَالَ لِرَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ: عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلَاتِي، قَالَ: ((قُلْ: اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا (وَفِي رِوَايَةٍ كَبِيرًا بَدَلَ كَثِيرًا) وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ))
It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with him) that Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him) said to the Messenger of Allah (peace and blessings be upon him): Teach me a supplication so that I may recite it in prayer. The Prophet (peace and blessings be upon him) said: Recite this supplication: "O Allah! Indeed, I have wronged myself greatly, and none forgives sins except You, so forgive me with a forgiveness from Yourself, and have mercy on me. Indeed, You are the Most Forgiving, Most Merciful."