Lying down with the last thing you say being belief
Taught to al-Barā', with the instruction that if he died that night, he would die upon the natural way.
إِذَا أَتَيْتَ مَضْجَعَكَ فَتَوَضَّأْ وُضُوءَكَ لِلصَّلاَةِ، ثُمَّ اضْطَجِعْ عَلَى شِقِّكَ الأَيْمَنِ، ثُمَّ قُلِ اللَّهُمَّ أَسْلَمْتُ وَجْهِي إِلَيْكَ، وَفَوَّضْتُ أَمْرِي إِلَيْكَ، وَأَلْجَأْتُ ظَهْرِي إِلَيْكَ، رَغْبَةً وَرَهْبَةً إِلَيْكَ، لاَ مَلْجَأَ وَلاَ مَنْجَا مِنْكَ إِلاَّ إِلَيْكَ، اللَّهُمَّ آمَنْتُ بِكِتَابِكَ الَّذِي أَنْزَلْتَ، وَبِنَبِيِّكَ الَّذِي أَرْسَلْتَ. فَإِنْ مُتَّ مِنْ لَيْلَتِكَ فَأَنْتَ عَلَى الْفِطْرَةِ، وَاجْعَلْهُنَّ آخِرَ مَا تَتَكَلَّمُ بِهِ
Allāhumma āmantu bi-kitābika-lladhī anzalt, wa bi-nabiyyika-lladhī arsalt.
“O Allah, I believe in Your Book which You revealed, and in Your Prophet whom You sent.”
He was told to make these the last words he spoke. Not the last of his prayer, the last of his day.
The full narration
Narrated Al-Bara 'bin `Azib:The Prophet (ﷺ) said to me, "Whenever you go to bed perform ablution like that for the prayer, lie on your right side and say, "Allahumma aslamtu wajhi ilaika, wa fauwadtu `Amri ilaika, wa alja'tu Zahri ilaika raghbatan wa rahbatan ilaika. La Malja'a wa la manja minka illa ilaika. Allahumma amantu bikitabika-l-ladhi anzalta wa bina-biyika-l ladhi arsalta" (O Allah! I surrender to You and entrust all my affairs to You and depend upon You for Your Blessings both with hope and fear of You. There is no fleeing from You, and there is no place of protection and safety except with You O Allah! I believe in Your Book (the Qur'an) which You have revealed and in Your Prophet (Muhammad) whom You have sent). Then if you die on that very night, you will die with faith (i.e. on the religion of Islam). Let the aforesaid words be your last utterance (before sleep)." I repeated it before the Prophet (ﷺ) and when I reached "Allahumma amantu bikitabika-l-ladhi anzalta (O Allah I believe in Your Book which You have revealed)." I said, "Wa-rasulika (and your Apostle)." The Prophet (ﷺ) said, "No, (but say): 'Wanabiyika-l-ladhi arsalta (Your Prophet whom You have sent), instead
Source
Ṣaḥīḥ al-Bukhārī · 247
- Agreed upon as authentic (al-Bukhārī / Muslim)