Authentic Supplication Number 101.

It is reported by way of `Abdullah son of Amru  that Abu Bakr As-siddeeq  said to the Messenger of Allah : Teach me an invocation [a supplication] which I may petition [Allah] with in my prayer, he said: “Say: ‘Allahumma ‘innee thalamtu nafsee thulman katheeran, wa la yaghfi ruth-thunooba ‘illa ‘anta, faghfirlee maghfiratan min ‘indika, warhamnee, ‘innaka ‘antal ghafoorur-raheemu (119).”

(119) O’ my ‘ilah I have transgressed upon myself much transgression, and no one forgives sins except for You, so forgive me a forgiveness from You, and have mercy upon me. You are The Oft-Forgiving. The Merciful.

Agreed upon by Bukhari and Muslim, and is #84 in The Authentic of Good Sayings.