Musnad Ahmad 658:48

'Affan narrated to us, Shu'bah narrated to us, from Abu Ishaq, who said: I heard Al-Bara' ibn 'Azib [saying] that the Messenger of Allah (may Allah bless him and grant him peace) commanded a man from the Ansar to say when he took to his bed: "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 leaned my back upon You, out of desire and fear of You. There is no refuge and no escape from You except to You. I have believed in Your Book which You revealed, and Your Prophet whom You sent." If he dies, he dies upon the Fitrah (natural disposition).