Sahih Muslim Book 4, Hadith Number 1694.

Chapter : Encouragement to observe prayers during Ramadhan and that is Tarawih.

‘Abd al-Rahman b. ‘Auf reported: I asked ‘Aisha, the mother of the believers, (to tell me) the words with which the Apostle of Allah (may peace be upon him) commenced the prayer when he got up at night. She said: When he got up at night he would commence his prayer with these words: O Allah, Lord of Gabriel, and Michael, and Israfil, the Creator of the heavens and the earth, Who knowest the unseen and the seen; Thou decidest amongst Thy servants concerning their differences. Guide me with Thy permission in the divergent views (which the people) hold about Truth, for it is Thou Who guidest whom Thou wilt to the Straight Path.