Musannaf Ibn Abi Shayba 21:175

Abu al-Ahwas narrated to us, from Abu Ishaq, from al-Bara’, who said: The Messenger of Allah (may Allah bless him and grant him peace) said to a man: "O so-and-so, when you go to your bed, say: 'O Allah, I have submitted my soul to You, and I have turned my face toward You, and I have turned my back to You.'" Then he mentioned the rest of it similarly, except that he said: "For if you die during your night, you will die upon the natural disposition (fitrah), and if you wake up, you will wake up to good."