In The Name of Allaah, The Most Merciful, The Bestower of Mercy
The Messenger of Allah said to Abay, “O Abu Mundhir! Do you know which verse from the Book of Allah – [you have memorised] – is the greatest?” He [Ubay] said, “Allaah and His Messenger know best.” The Prophet [repeated and] said, “O Abu Mundhir [Ubay]! Do you know which verse from the Book of Allaah – [you have memorised] – is the greatest?”. He [Ubay] said, “Allāhu laa ilāha illa huwal-Hayyul-Qayyoom [Aayatul-Kursi]. He [Ubay] said, “He [The Prophet] then struck me on my chest and said “By Allah, may you be blessed in your knowledge O Aba Mundhir.” [Muslim, hadith no.810]
Firstly: Ponder upon the question Ubay [radiyallaahu anhu] was asked!
He [radiyallaahu anhu] was a haafidh of the Qur’aan and was called the leader of the reciters. The Aayaat of the Qur’aan exceed six thousand in number, so when the Prophet asked him which Aayaah of the Qur’aan is the greatest, he extracted it from that [huge] number. He did not extract it from ten, a hundred or two hundred Aayaat, rather he extracted it from over six thousand Aayaat.
Secondly: Ponder upon the fact also that Ubay answered the question immediately. It did not take him a week, two weeks, a month or two, rather he answered immediately. When the Prophet [sallal laahu alayhi wasallam] asked him at first, he said, “Allaah and his Messenger know best.” But when the Prophet [sallal laahu alayhi wasallam] repeated the question, he knew that the Prophet [sallal laahu alayhi wasallam] permitted him to strive and find the answer, so he replied.
Thirdly: Also ponder upon how fast Ubay [radiyallaahu anhu] answered, which shows us the great station of Tawheed in the hearts of the Sahaabah and that Tawheed is the greatest subject matter in the Qur’aan. That is because when Ubay [radiyallaahu anhu] was asked he did not quote an Aayah regarding rulings on other matters nor an Aayaah regarding the story of the nations of the past, nor an Aayah regarding the events on the Day of Judgement, nor Aayaat regarding manners, rather the first thing that came to his mind was Tawheed.