The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1769
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.