It is narrated from Sayyiduna Abu Malik al-Ash'ari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The most complete words with Allah are that the servant says: O Allah, You are my Lord and I am Your servant. I have wronged myself and I confess my sin, and none forgives sins except You. My Lord, forgive me."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3436