Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: After the takbir (of entering the prayer), and after saying: "I have turned my face towards the One who created the heavens and the earth, as one inclining to the truth, as a Muslim," the Messenger of Allah (peace and blessings be upon him) used to say this supplication: "O Allah, to You alone belongs all praise; You are the Light of the heavens and the earth and whatever is in them. You are the Truth, Your promise is true, the meeting with You is true, Paradise is true, Hell is true, the Hour is true. O Allah, to You alone have I submitted, in You alone have I believed, upon You alone have I relied, to You alone have I turned, by Your help alone have I disputed (against falsehood), and to You alone have I brought my judgement. So forgive me what I have done before and what I have left after, what I have concealed and what I have done openly (all my sins). O Allah, You alone are my God; there is none worthy of worship except You." Then, after this, the Messenger of Allah (peace and blessings be upon him) would recite (the Qur'an).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10947