Abu Dawud Book 25, Hadith Number 3992.

Narated By Ibn al-Asqa’ : The Prophet (PBUH) came to them in the dwelling place of the immigrants and a man asked him: Which is the greatest verse of the Qur’an? The Prophet (PBUH) replied: “Allah, there is no god but He – the Living, the Self-Subsisting Eternal. No slumber can seize Him nor sleep.”(Quran ii 255)