Jami` at-Tirmidhi 3531

Abdullah bin `Amr narrated from Abu Bakr As-Siddiq that he said:“O Messenger of Allah, teach me a supplication that I may supplicate with in my Salat.” He (ﷺ) said: “Say: ‘O Allah, I have wronged myself much, and none forgives sins except You. So forgive me with forgiveness from You, and have mercy upon me, indeed, You are the Forgiving, the Merciful (Allāhumma innī ẓalamtu nafsī ẓulman kathīran wa lā yaghfirudh-dhunūba illā anta faghfirlī maghfiratan min `indika warḥamnī innaka antal-Ghafūrur-Raḥīm).’”