Musnad Ahmad 213:0

Narrated to us by Yahya ibn Sa'id, from Sufyan, who said: Narrated to me Abu Ishaq, from al-Bara' ibn 'Azib, who said: The Messenger of Allah (ﷺ) said: "When you go to your bed, perform ablution as you perform it for prayer, then lie down on your right side, then say: 'O Allah, I have submitted my soul to You, and I have turned my face toward You, and I have entrusted my affair to You, and I have relied upon You, out of hope for You and fear of You. There is no refuge nor escape from You except to You. I have believed in Your Book which You revealed, and in Your Prophet whom You sent.' If you die that night, you will die upon the natural disposition (fitrah). And let these be the last words you speak."