Sahih Muslim Book 4, Hadith Number 1768.

Chapter : Excellence of sura Fatiha and concluding verses of surat al-Baqara and exhortation to recite the last two verses of surat al-Baqara.

Ubayy b. Ka’b said: Allah’s Messenger (may peace be upon him) said: O Abu’ al-Mundhir, do you know the verse from the Book of Allah which, according to you, is the greatest? I said: Allah and His Apostle (may peace be upon him) know best. He again said: Abu’l-Mundhir, do you know the verse from the Book of Allah which, according to you, is the greatest? I said: Allah, there is no god but He, the Living, the Eternal. Thereupon he struck me on my breast and said: May knowledge be pleasant for you, O Abu’l-Mundhir!