The Prophet (ﷺ) said, "If you want to go to your bed, then perform ablution, lie on your right side and then supplicate:

اَللَّهُمَّ أَسْلَمْتُ نَفْسِي إِلَيْكَ، وَفَوَّضْتُ أَمْرِي إِلَيْكَ، وَوَجَّهْتُ وَجْهِيْ إِلَيْكَ، وَأَلْجَأْتُ ظَهْرِي إِلَيْكَ، رَغْبَةً وَرَهْبَةً إِلَيْكَ، لَا مَلْجَأَ وَلَا مَنْجَا مِنْكَ إِلَّا إِلَيْكَ، آمَنْتُ بِكِتَابِكَ الَّذِي أَنْزَلْتَ، وَبِنَبِيِّكَ الَّذِي أَرْسَلْتَ

Allaahumma 'aslamtu nafsee 'ilayka, wa fawwadhtu 'amree 'ilayka, wa wajjahtu wajhee 'ilayka, wa 'alja'tu dhahree 'ilayka, raghbatan wa rahbatan 'ilayka, laa maalja' wa laa manjaa minka 'illaa 'ilayka, 'aamantu bikitaabikal-ladhee 'anzalta wa bi-nabiyyikal-ladhee 'arsalta

O Allah, I submit my soul unto You, and I entrust my affair unto You, and I turn my face towards You, and I totally rely on You, in hope and fear of You. Verily there is no refuge nor safe haven from You except with You. I believe in Your Book which You have revealed and in Your Prophet whom You have sent. The Prophet (ﷺ) said to the person to whom he taught this Dua: If you die that night, you die on Fitrat, the religion of Islam.

Reference: Al-Bukhari, Fathul Bari 11/113 no. 6313, Muslim 4/2081 no. 2710