O Allah! I have considerably wronged myself​

I requested the Messenger of Allah may the peace and blessings of Allah be upon him,  to teach me a supplication which I could recite in my Salat (prayer). Thereupon he said, "Recite: 'Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh- dhunuba illa Anta, faghfir li maghfiratan min 'indika, warhamni, innaka Antal-Ghafur-ur-Rahim (O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate)."