Someone said to the Messenger of Allah ﷺ, "Teach me a supplication to recite in my prayer."He replied, "Say, 'O Allah, I have wronged myself greatly, and none forgives sins but You. So grant me forgiveness from Yourself and have mercy on me. Truly, You are the Forgiving, the Merciful.'"
حدثنا محمد بن رمح حدثنا الليث بن سعد عن يزيد بن أبي حبيب عن أبي الخير عن عبد الله بن عمرو بن العاص
لرسول الله ﷺ علمني دعاء أدعو به في صلاتي قال قل اللهم إني ظلمت نفسي ظلما كثيرا ولا يغفر الذنوب إلا أنت فاغفر لي مغفرة من عندك وارحمني إنك أنت الغفور الرحيم
Bukhari narrated it in the Book of Adhan, Chapter on Supplication Before the Salutation (Taslim), no. 834, from the hadith of Layth, through this route. Muslim narrated it in the Book of Remembrance and Supplication, Chapter on Supplications and Seeking Refuge, no. 48/2705, from Ibn Ramh, through this route.