The Greatest Ayah in the Qur’an
Ubayy bin Ka’b (radi Allahu anhu) reported: The Messenger of Allah (sal Allahu alaihi wa sallam) said: “Abu Mundhir! Do you know which Ayah in Allah’s Book is the greatest?” I replied: “It is ‘Allah la ilaha illa Huwal-Haiyul-Qayum (Allah! none has the right to be worshipped but He, the Ever Living…)’.” (2:256) Thereupon He (sal Allahu alaihi wa sallam) patted me on the chest and said, “Rejoice by this knowledge, O Abu Mundhir!”
The words cited in the Hadith occur at the beginning of Ayat-ul-Kursi (No. 256 of Surat Al-Baqarah of the Qur’an).
The merits mentioned in the Hadith are for the whole Ayah. Ayat-ul-Kursi mentions the Majestic Attributes and Mighty Powers of Allah and is, therefore, very meritorious.
“Rejoice by this knowledge” means “May this knowledge be a source of respect, honour and benefit to you”. Knowledge here stands for the knowledge of the Qur’an and Hadith which is certainly a means of success in this world and the Hereafter.
And Allah knows best!