How to protect yourself while sleeping?.

 One of the virtues of Ayat Al-Kursi is that it keeps Satan away from you, where as narrated by Abu Hurayrah: "Allah's Messenger deputed me to keep the Zakah of Ramadan (The charity) of Fitr), a comer came and started taking handfuls of the foodstuff, I took hold of him and said By Allah! I will take you to Allah's Messenger (PBUH), he said I am needy and have many dependents, and I am in great need, so I released him, and in the morning Allah's Messenger asked me What did your prisoner do yesterday?, I said O Allah's Messenger! The person complained of being needy and of having many dependents, so I pitied him and let him go, Allah's Messenger said indeed he told you a lie and he will come again, so I believed that he would come again as Allah's Messenger had told me that he would come again. So I waited for him watchfully, when he started stealing handfuls of foodstuff, I caught hold of him again and said I will definitely take you to Allah's Messenger, he said Leave me for I am very needy and have many dependents, I promise I will not come back again, so I pitied him and let him go. 

 In the morning, Allah's Messenger (PBUH) asked me What did your prisoner do?, I replied O Allah's Messenger! He complained of his great need and of too many dependents, so I took pity n him and set him free, Allah's Messenger said indeed, he told you a lie and he will return, I waited for him attentively for the third time, and when he started stealing handfuls of the foodstuff, I caught hold of him and said I will surely take you to Allah's Messenger as it is the third time you promise not to return, yet you break your promise and come, he said Let me and I will teach you some words with which Allah will benefit you, I asked What are they?, he replied whenever you go to bed Recite {Allahu la ilaha ella huwa alhayyu alqayyumu: Allah, there is no God but He, the Ever-Living, the Sustainer of existence} till you finish the whole verse, if you do so, Allah will appoint a guard for you who will stay with you and no Satan will come near you till morning, so I released him. 

 In the morning, Allah's Messenger asked me What did your prisoner do yesterday?, I replied he claimed that he would teach me some words by which Allah will benefit me, so I let him go, Allah's Messenger asked What are they?, I replied he said to me whenever you go to bed, Recite Ayat Al-Kursi from the beginning to the end  {Allahu la ilaha ella huwa alhayyu alqayyumu: Allah, there is no God but He, the Ever-Living, the Sustainer of existence}, he further said to me if you do so, Allah will appoint a guard for you who will stay with you and no Satan will come near you till morning, Abu Hurayrah or another sub-narrator added that they were very keen to do good deeds, the Prophet (PBUH) said he really spoke the truth, although he is an absolute lair, D o you know whom you were talking to these three nights O Abu Hurayrah?, Abu Hurayrah said no, He (PBUH) said it was Satan" (Sahih Albukhari 2311).

what is Ayat Al-Kursi?

 Ayat Al-Kursi is Aya number 255 of Surat Albaqarah and it consists of ten sentences, and it is the greatest Ayah in the Holy Quran, where as narrated by Ubayy bin Ka'b that the Messenger of Allah (PBUH) said: "O Abu Al-Mundhir! Do you know the verse from the Book of Allah which, according to you, is the greatest?, I said Allah and His Messenger know best, He again said O Abu Al-Mundhir! Do you know the verse from the Book of Allah which, according to you, is the greatest?, I said Allah, there is no God but He, the Ever-Living, the Sustainer of existence, thereupon, He (PBUH) struck me on my breast and said may knowledge be pleasant for you O Abu Al-Mundhir" (Sahih Muslim 810).

Ayat Al-Kursi: 

{Allahu la ilaha ella huwa alhayyu alqayyumu la ta'khuzuhu senatun wa la nawmun lahu ma fee alsamawati wa ma fee al-ardi man za allazi yashfa-ou endahu ella be-iznehi ya'lamu ma baiyna aiydehim wa ma khalfahum wa la youheetuna be-shaiy-in men elmehi ella be-ma sha-a wase-a kursiyyuhu alsamawati wa al-arda wa la ya-uduhu hefzuhuma wa huwa Al-aliyyu Al-azeem}.

Its meaning:

Allah, there is no God but He, the Ever-Living, the Sustainer of existence, neither drowsiness overtakes Him nor sleep, to Him belongs whatever is in the heavens and whatever is on the earth, Who is the one who can intercede with Him except by His permission?, He knows what is before them and what is behind them, and they do not encompass anything of His knowledge except for what He wills, His Chair extends to the heavens and the earth, and guarding both of them does not tire Him, and He is the Most High, the Most Great.