He asked him three times, but Ubayy out of adab (politeness) replied that Allah and His messenger knew best, then on the third time, Ubayy ibn Ka’b reported that the Messenger of Allah ﷺ said:

The Prophet (peace be on him) asked Ubayy bin Ka’b who was one of the top reciters of the Quran and a hafidh (the one who had memorised the Quran) which ayah was the best. Ayatul Kursi is the greatest ayah in the Quran Until one day, someone sees it and points out that it is absolutely priceless! And those who inherited it, are surprised because they had no idea, it just fell into their laps without them appreciating it. A treasure which many people have inherited without having a clue about its real value. Is the Ayatul Kursi a du’a? Yes, it does fall in that category. It is from the sunnah to recite the Ayatul Kursi. If you imagine your day, when you wake up or when you are going to sleep, reciting the Ayatul Kursi should be part of our routine. His Kursi extends to the Heavens and to the Earth, and it does not weary Him to look after them. Who can intercede with Him without His permission? He knows what is before them and what is behind them while they encompass nothing of His knowledge, except what He wills. To Him belongs all that is in the heavens and all that is on the earth. The Ayatul Kursi, the 255th ayah of Surat al Baqarah is not just the greatest ayah, but comes from the greatest and longest surah in the Quran.

Surah kursi translation