Sahih Muslim Book 4, Hadith Number 1769.

Chapter : Excellence of surat al-Kahf and Ayat al-Kursi.

Abu Darda’ reported Allah’s Apostle (may peace be upon him) as saying: Is any one of you incapable of reciting a third of the Qur’an in a night? They (the Companions) asked: How could one recite a third of the Qur’an (in a night)? Upon this he (the Holy Prophet) said: “He is Allah, One” (Qur’an. cxii.) is equivalent to a third of the Qur’an.