Sahih Bukhari Volume 6, Book 61, Hadith Number 534.

Narrated By Abu Said Al-Khudri : The Prophet said to his companions, “Is it difficult for any of you to recite one third of the Qur’an in one night?” This suggestion was difficult for them so they said, “Who among us has the power to do so, O Allah’s Apostle?” Allah Apostle replied: ” Allah (the) One, the Self-Sufficient Master Whom all creatures need.’ (Surat Al-Ikhlas 112.1 to the End) is equal to one third of the Qur’an.”