Duʿāʾ of seeking forgiveness taught to Abū Bakr to be said during Ṣalāh
that he said to the Messenger of Allah ﷺ, teach me a duʿāʾ that I may say in my prayer. He said: "Say: O Allah, I have wronged myself greatly and no one forgives sins but you, grant me forgiveness from you and have mercy on me for you are the Oft Forgiving Most Merciful."
أنَّهُ قالَ لِرَسُولِ اللهِ ﷺ: عَلِّمْنِي دُعاءً أدْعُو بِهِ فِي صَلاتِي. قالَ: «قُلِ: اللهُمَّ إنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا، ولا يَغْفِرُ الذُّنُوبَ إلّا أنْتَ، فاغْفِرْ لِي مَغْفِرَةً مِن عِنْدِكَ، وارْحَمْنِي، إنَّكَ أنْتَ الغَفُورُ الرَّحِيمُ»