Hadith is about the Greatest Ayah

Ubayy bin Ka`b RA reported: The Messenger of Allah (sallallaahu ’alayhi 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 (sallallaahu ’alayhi wa sallam) patted me in the chest and said, "Rejoice by this knowledge, O Abu Mundhir!" (Muslim)