Hadith: O Abu al-Mundhir, do you know which verse of Allah's Book that you have is the greatest?" I said: "{Allah: none has the right to be worshiped except Him, the Ever-Living, All-Sustaining...} [Surat al-Baqarah: 255]." Thereupon, he struck me on the chest and said: "May knowledge be pleasant for you, O Abu al-Mundhir!

عَنْ ‌أُبَيِّ بْنِ كَعْبٍ رضي الله عنه قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«يَا أَبَا الْمُنْذِرِ، أَتَدْرِي أَيُّ آيَةٍ مِنْ كِتَابِ اللهِ مَعَكَ أَعْظَمُ؟» قَالَ: قُلْتُ: اللهُ وَرَسُولُهُ أَعْلَمُ. قَالَ: «يَا أَبَا الْمُنْذِرِ، أَتَدْرِي أَيُّ آيَةٍ مِنْ كِتَابِ اللهِ مَعَكَ أَعْظَمُ؟» قَالَ: قُلْتُ: {اللهُ لا إِلَهَ إِلا هُوَ الْحَيُّ الْقَيُّومُ} [البقرة: 255]. قَالَ: فَضَرَبَ فِي صَدْرِي، وَقَالَ: «وَاللهِ لِيَهْنِكَ الْعِلْمُ، أَبَا الْمُنْذِرِ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 810]

Ubayy ibn Ka‘b (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said:
"O Abu al-Mundhir, do you know which verse of Allah's Book that you have is the greatest?" I replied: "Allah and His Messenger know best." He said: "O Abu al-Mundhir, do you know which verse of Allah's Book that you have is the greatest?" I said: "{Allah: none has the right to be worshiped except Him, the Ever-Living, All-Sustaining...} [Surat al-Baqarah: 255]." Thereupon, he struck me on the chest and said: "May knowledge be pleasant for you, O Abu al-Mundhir!"
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) asked Ubayy ibn Ka‘b about the greatest verse in Allah's Book. He initially hesitated about the answer and then said: {Allah: none has the right to be worshiped except Him, the Ever-Living, All-Sustaining...} The Prophet (may Allah's peace and blessings be upon him) agreed with him and struck him on the chest, indicating that it is filled with knowledge and wisdom. He also supplicated that this knowledge be pleasant and easy for him.