I asked Allâh's Messenger (ﷺ) to teach me an invocation so that I may invoke Allâh with it in my (prayer). He told me to say,

"Allâhumma innî zalamtu nafsi zulman kathîran. Walâ yaghfirudh-dhunuba illâ anta faghfirlî maghfiratan min indika, warḥamnî innaka antal-Ghafûr ur- Rahim

[O Allâh ! I have done great Zulm (wrong) to myself and none except You forgives sins, so please forgive me and be Merciful to me as You are the Oft-Forgiver, the Most Merciful]." [1:796-O.B.]