Musnad Ahmad 658:94

Waki’ narrated to us, Fitr narrated to us, from Sa’d ibn ‘Ubaydah, from al-Bara’ ibn ‘Azib, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) went on fifteen military expeditions." Waki’ narrated to us, Fitr narrated to us, from Sa’d ibn ‘Ubaydah, from al-Bara’ ibn ‘Azib, that the Messenger of Allah (peace and blessings of Allah be upon him) said to a man: "When you go to your bed in a state of purity, say: 'O Allah, I have submitted my face to You, and I have entrusted my back to You, and I have committed my affair to You, out of desire and fear of You. There is no refuge and no escape from You except to You. I believe in Your Book which You have revealed, and Your Prophet whom You have sent.' If you die that night, you will die upon the natural disposition (fitrah), and if you wake up, you will wake up having attained much good." ‘Abdullah said: My father said: "Fitr heard it from Sa’d ibn ‘Ubaydah."