اَللَّهُ لَآ إِلٰهَ إِلَّا هُوَ الْحَىُّ الْقَيُّومُ ۚ لَا تَأْخُذُهُۥ سِنَةٌ وَلَا نَوْمٌ ۚ لَّهُۥ مَا فِى السَّمٰوٰتِ وَمَا فِى الْأَرْضِ ۗ مَنْ ذَا الَّذِى يَشْفَعُ عِنْدَهُۥٓ إِلَّا بِإِذْنِهِۦ ۚ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ ۖ وَلَا يُحِيطُونَ بِشَىْءٍ مِّنْ عِلْمِهِۦٓ إِلَّا بِمَا شَآءَ ۚ وَسِعَ كُرْسِيُّهُ السَّمٰوٰتِ وَالْأَرْضَ ۖ وَلَا يَـُٔودُهُۥ حِفْظُهُمَا ۚ وَهُوَ الْعَلِىُّ الْعَظِيمُ

Allahu laa 'ilaaha 'illaa Huwal-Hayyul-Qayyoom, laa ta'khudhuhu sinatun wa laa nawm, lahu maa fis-samaawaati wa maa fil-'ardh, man dhal-ladhee yashfa'u 'indahu 'illaa bi'idh nih, ya'lamu maa bayna 'aydeehim wa maa khalfahum, wa laa yuheetoona bishay'im-min 'ilmihi 'illaa bimaa shaa'a, wasi'a kursiyyuhus-samaawaati wal'ardh, wa laa ya'ooduhu hifdhuhumaa, wa Huwal-'Aliyyul- 'Adheem

Allah! There is none worthy of worship but He, the Ever Living, the One Who sustains and protects all that exists. Neither slumber nor sleep overtakes Him. To Him belongs whatever is in the heavens and whatever is on the earth. Who is he that can intercede with Him except with His Permission? He knows what happens to them in this world, and what will happen to them in the Hereafter. And they will never compass anything of His Knowledge except that which He wills. His Throne extends over the heavens and the earth, and He feels no fatigue in guarding and preserving them. And He is the Most High, the Most Great. (Sura Baqarah 2:255) Whoever reads Ayatul Kursi when he lies down to sleep will have a guardian from Allah remain with him and Satan will not be able to come near him until he rises in the morning.

Reference: Al-Bukhari, Fathul Bari 4/487, no. 2311