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».

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!"

Source · Narrated by Muslim

From the Hadith Encyclopedia (hadeethenc.com).