Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1455
Narated By Ubayy b. K'ab : The Apostle of Allah (pbuh) said: Abu al-Mundhir, which verse of the Allah's Book that you have is greatest? I replied: Allah and His Apostle know best. He said: Abu al-Mundhir, which verse of the Allah's Book that you have is greatest? I said: Allah, there is no god but He, the Living, the Eternal. Thereupon he struck me on the breast and said: May knowledge be pleasant for you, Abu al-Mundhir.