Imam Ahmad recorded that Ubayy bin Ka`b said that;
The Prophet 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 several times, Ubayy said, "Ayat Al-Kursi. The Prophet 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 with which it praises Allah next to the Throne. [Imam Ahmad , Muslim] This Hadith was also collected by Muslim, but he did not include the part that starts with, "By He in Whose Hand".
“Whoever recites Ayat ul-Kursi immediately after each prescribed Prayer, there will be nothing standing between him and his entering Paradise except death.” [Narrated by an-Nasaaee. Imaam Al-Albaanee authenticated it in Saheeh al Jam i as-Sagheer : 6464]