Ubayy bin Ka'b (May God be pleased with him) narrates:

The Prophet (Peace and Blessings be Upon Him) said:

“O Abu'l-Mundhir, do you know the verse from the Book of God which, in your memory, is the greatest?”

I said: “Allah, there is no god but He, the Living, the Eternal…” (Surah al-Baqara,255) and this is called the Ayatul Kursi.

Thereupon he struck me on my chest and said: “May knowledge be pleasant for you, O Abu'l-Mundhir!”

(Muslim, Guests 258)