Salaam and Greetings of Peace:
Once a man came into the Mosque of the Prophet, lamenting over his sins. The man was so overcome and saddened by his state that the Prophet [pbuh] said to him, “Say this:”
“Oh Allah, Your forgiveness is greater than my sins, and my hope in Your mercy is greater to me than my actions”
The man said this and the Prophet [pbuh] said to him, “Say it again.” The man repeated it, then the Prophet (pbuh) said to him, “Say it again.” The man did so a third time, and the Prophet [pbuh] said to him, “Arise, for indeed Allah has forgiven you.”
– This hadith was related by al-Hakim with a sound chain that he considered a match for the standards of Imam Muslim. Taken from a longer post on the Suhaib Webb blog (where the Arabic is included), and with thanks to Ali Eteraz for pointing the way.