Reference : Hisn al-Muslim 100

((اللَّهُ لاَ إِلَهَ إِلاَّ هُوَ الْحَيُّ الْقَيُّومُ
لاَ تَأْخُذُهُ سِنَةٌ وَلاَ نَوْمٌ
لَّهُ مَا فِي السَّمَوَاتِ وَمَا فِي الأَرْضِ
مَن ذَا الَّذِي يَشْفَعُ عِنْدَهُ إِلاَّ بِإِذْنِهِ
يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ
وَلاَ يُحِيطُونَ بِشَيْءٍ مِّنْ عِلْمِهِ إِلاَّ بِمَا شَاء
وَسِعَ كُرْسِيُّهُ السَّمَوَاتِ وَالأَرْضَ
وَلاَ يَؤُودُهُ حِفْظُهُمَا
وَهُوَ الْعَلِيُّ الْعَظِيمُ))
Allāhu lā ilāha illā huwa ‘l-Ḥayyul-Qayyūm,
lā ta'khudhuhu sinatun wa lā nawm,
lahu mā fis-samāwāti wa māfil-ardh,
man dhal-ladhī yashfa`u `indahu illā bi'idhnih,
ya`lamu mā bayna aydīhim wa mā khalfahum,
wa lā yuḥītūna bishay'im-min `ilmihi illā bimā shā',
wasi`a kursiyyuhus-samāwāti wal-ardh,
wa lā ya'ūduhu hifẓuhumā,
wa huwal-`Aliyyu‘l-`Aẓīm.

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 encompass 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.

Reference:
Al-Baqarah 2:255. Whoever reads this 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. See Al-Bukhari, cf. Al-Asqalani, 
Fathul-Bari 4/487.

Chapter: Before sleeping