He 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. You are indeed the Forgiving, the Merciful.'"Yunus said "kabiran" (greatly). Hasan al-Ashyab narrated it to us from Ibn Lahiah, who said, "kabiran."
حدثنا هاشم بن القاسم قال حدثنا الليث قال حدثني يزيد بن أبي حبيب عن أبي الخير عن عبد الله بن عمرو
أنه قال 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 وقال يونس كبيرا حدثناه حسن الأشيب عن ابن لهيعة قال قال كبيرا
Its isnad is sahih. Abul-Khair is Marthad bin Abdullah al-Yazani. Then Imam Ahmad narrated this hadith right after it with another chain, which he did not complete, but it is clear. He narrated it from Hasan al-Ashyab, from Ibn Lahia, meaning from Yazid bin Abu Habib, and so on. In H it appears as "from Abu Wahb Lahia," which is an error.