Sahih Muslim Book 4, Hadith Number 1696.

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

A’raj reported that when the Messenger of Allah (may peace be upon him) would start the prayer, he would pronounce takbir (Allah-o-Akbar) and then say: I turn my face (up to Thee), I am the first of the believers; and when he raised his head from ruku’ he said: Allah listened to him who praised Him; O our Lord, praise be to Thee; and he said: He shaped (man) and how fine is his shape? And he (the narrator) said: When he pronounced salutation he said: O Allah, forgive me my earlier (sins), to the end of the Hadith; and he did not say it between the Tashahhud and salutation (as mentioned above).