Sayyiduna Abu Rafi' (may Allah be pleased with him) said: A written document came to me in which the opening supplication (du'a al-istiftah) of the Messenger of Allah (peace and blessings be upon him) was recorded. When he (peace and blessings be upon him) said the takbir, he would say: "I have turned my face, as one inclining to the truth, towards the One who created the heavens and the earth, and I am not of the polytheists. Indeed, my prayer, my rites of worship, my living and my dying are all for Allah, the Lord of the worlds; He has no partner, and with this I have been commanded, and I am the first of the Muslims. O Allah, You are the King; there is no deity except You. You are free of all imperfection, and all praise is Yours. You are my Lord and I am Your servant. I have wronged myself and I confess my sin, so forgive me all my sins, for none forgives sins except You. Here I am at Your service, here I am at Your service, and all good is in Your hands. There is no refuge and no escape from You except in turning to You. I seek Your forgiveness and I turn to You in repentance." Then he (peace and blessings be upon him) would recite.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 924