The Prophet of Allah 
said: "When you are about to sleep recite ayat-al-kursi till the end of the verse for there will remain over you a protection from Allah and no devil will draw near to you until morning.
Ayat Al-kursi is found in the Quran
Surah Al-Baqara (The Cow) - Chapter 2 Verse 255
The importance of Ayat Al-kursi
- Ubayy bin Ka'b  reported: The Messenger of Allah  said: "Abu Mundhir! Do you know which Ayah in Allah's Book is the greatest? I said: 'Allah and His messenger know best.'
He  again said: 'Do you know which Ayah in Allah's Book, according to you, is the greatest?' I (Abu Mundhir) replied: ‘It is 'Allah la ilaha illa Huwal-Hayyul-Qayyum'.[2:255].
Thereupon he  patted me in the chest and said, 'Rejoice by your knowledge, O Abu Mundhir! (i.e. may this knowledge be a source of respect, honour and benefit to you)'."