It is narrated from Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) that he said to the Messenger of Allah (peace and blessings be upon him): Teach me a supplication that I may make in my prayer. He (peace and blessings be upon him) said: "Say: 'Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh-dhunuba illa anta, faghfir li maghfiratan min 'indika warhamni, innaka antal-ghafurur-rahim' (O Allah, I have greatly wronged myself, and none forgives sins except You, so forgive me with a forgiveness from Yourself and have mercy on me; indeed, You are the Most Forgiving, the Most Merciful.)"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2884