Ayat-ul-Kursi is described as the greatest Ayah in the Quraan.
Imam Ahmad recorded that 'Ubayy bin Ka'b said that the Prophet (sal) asked him about the greatest Ayah in the Book of Allah, and 'Ubayy answered "Allah and his Messenger know better." When the Prophet repeated his question serveral times, Ubayy said "Ayat-ul-Kursi" The Prophet (sal) commented, "Congratulations for having knowledge, O Abu Al-Mundhir! By He in whose Hand is my soul! this Ayah has a tongue and two lips which she praises the King (Allah) next to the leg of the throne."
[Ahmad 5:14]
[Muslim 1:556 ]
Kursi : literally means a footstool or chair, and sometimes wrongly translated as Throne. the Kursi mentioned in this Verse should be distinguished from the Arsh (Throne) mentioned in (7:58, 10:3, 85:15 and elsewhere).
Ad-Dahhak said that Ibn Abbas said:
"If the seven heavens and the seven earths were flattened and laid side by side, they would add up to the size of a ring in a desert compared to the Kursi ."
[Ibn Abi Hatim 3:981]
Al Bukhari recorded the following hadith in his Sahih from Abu Huraira, in the chapter on the vitrues of the Quraan
Narrated Abu Huraira:
Allah's Apostle deputed me to keep Sadaqat (Al-Fitr) of Ramadan. A someone came and started taking handfuls of the foodstuff (of the Sadaqa) stealthily. I took hold of him and said, "By Allah, I will take you to Allah's Apostle (sal) ." He said, "I am needy and have many dependents, and I am in great need." I released him.
In the morning Allah's Apostle (sal) asked me, "What did your prisoner do yesterday?" I said, "O Allah's Apostle (sal) ! The person complained of being needy and of having many dependents, so, I pitied him and let him go."
Allah's Apostle said, "Indeed, he told you a lie and he will be coming again."
I believed that he would show up again as Allah's Apostle (sal) had told me that he would return. So, I waited for him watchfully. When he (showed up and) started stealing handfuls of foodstuff, I caught hold of him again and said, "I will definitely take you to Allah's Apostle (sal) . He said, "Leave me, for I am very needy and have many dependents. I promise I will not come back again." I pitied him and let him go.
In the morning Allah's Apostle (sal) asked me, "What did your prisoner do." I replied, "O Allah's Apostle! He complained of his great need and of too many dependents, so I took pity on him and set him free."
Allah's Apostle (sal) said, "Verily, he told you a lie and he will return."
I waited for him attentively for the third time, and when he (came and) started stealing handfuls of the foodstuff, I caught hold of him and said, "I will surely take you to Allah's Apostle (sal) as it is the third time you promise not to return, yet you break your promise and come." He said, "(Forgive 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 "Ayat-al-Kursi"
'Allahu la ilaha illa huwa-l-Haiy-ul Qaiyum'
(Allah, there is none worthy of worship but He, the Ever Living the One Who sustains and protects all that exists.)
[Surah Al Baqarah Ayah 255 (Ayat-ul-Kursi) ]
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 Apostle (sal) asked, "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 Apostle (sal) 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 illa huwa-lHaiy-ul-Qaiyum".' 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 Huraira or another sub-narrator) added that they (the companions) were very keen to do good deeds.
The Prophet(sal) said, "He really spoke the truth, although he is an absolute liar. Do you know whom you were talking to, these three nights, O Abu Huraira?" Abu Huraira said, "No." He (sal) said, "It was Satan."
[Sahih Al-Bukhari Volume 3, Book 38, Number 505]