It is narrated from Sayyiduna al-Asqa' al-Bakri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) came to them in the Suffah of the Muhajirun, and a man asked: Which verse in the Qur'an is the greatest? The Prophet (peace and blessings be upon him) said: "Allah - there is no god except Him, the Ever-Living, the Sustainer of all existence. Neither drowsiness overtakes Him nor sleep" [Surah al-Baqarah: 255].
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 995