عَنْ أَبِي بَكْرٍ الصِّدِّيقِ رَضِيَ اللّهُ عَنْهُ: أَنَّهُ قَالَ لِرَسُولِ اللّهِ صَلّى اللّهُ عَلَيْهِ وسَلَّم: عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلاتِي، قَالَ: «قُلْ: اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا، وَلا يَغْفِرُ الذُّنُوبَ إِلّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي، إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ» مُتَّفَقٌ عَلَيْهِ. وَفِي رِوَايَةٍ: «وَفِي بَيْتِي» وَرُوِيَ: «ظُلْمًا كَثِيرًا» وَرُوِيَ «كَبِيرًا» بِالثَّاءِ الْمُثَلَّثَةِ وَبِالْبَاءِ الْمُوَحَّدَةِ، فَيَنْبَغِي أَنْ يُجْمَعَ بَيْنَهُمَا، فَيُقَالَ: كَثِيرًا كَبِيرًا.

Abu Bakr al-Siddiq (may Allah be pleased with him) reported that he said to the Messenger of Allah (peace and blessings be upon him): Teach me a supplication with which I may supplicate in my prayer. He said: "Say: O Allah, I have greatly wronged myself, and none forgives sins but You; so grant me forgiveness from You and have mercy upon me. Indeed, You are the Oft-Forgiving, the Most Merciful." Agreed upon (al-Bukhari and Muslim). In another narration: "and in my home." It is also narrated "much (kathiran)" and narrated "great (kabiran)" — with the three-dotted thaʾ and with the single-dotted baʾ — so it is best to combine them and say: "much and great (kathiran kabiran)."