Riyad us Saliheen: Hadith 1018

The Messenger of Allah (ﷺ) 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 (ﷺ) patted me in the chest and said, "Rejoice by this knowledge, O Abu Mundhir!"

[Muslim].