The Prophet, peace be upon him, said:

The greatest verse in the Book of Allah is: ‘Allah, there is no god except Him — the Living, the Eternal.

The Prophet, peace be upon him, said:

When you go to your bed, recite Ayatul Kursi: ‘Allah, there is no god except Him — the Living, the Eternal’ to the end, for then there will remain over you a guardian from Allah, and Shaitaan will not come near you until morning

[Al Bukhari]

Translation of the verse:
Allah, none has the right to be worshiped but He, the Ever-Living, the Self-Sustaining and Sustainer of all. No slumber can seize Him, nor sleep. To him belongs whatever is in the heavens and on earth. Who is he that can intercede with Him except with His permission? And He knows what will happen to His creatures in this world and and in the Hereafter. Nor shall they encompass anything of His Knowledge except that which He wills. His Kursi 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. [Surat al-Baqarah (2):255]
Additionally, [Whoever recites this ayah after every prayer, nothing prevents him from entering Paradise besides first having to die.]