Abu Darda' reported Allah's Apostle (sallallaahu alayhi wa sallam) 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: "'Say: He is Allah, One' (Qur'an Suurah al-'ikhlaas 112) is equivalent to a third of the Qur'an."
Sahiih Muslim Hadiith No. 811
Abu Huraira reported:
The Messenger of Allah (sallallaahu alayhi wa sallam) came out to us and said: I am going to recite before you one-third of the Qur'an. He (the Holy Prophet) then recited:" Say: He is Allah, One--Allah, the Eternal," to the end of the Surah.
Sahiih Muslim Hadiith No. 812
Narrated Abu Sa`id Al-Khudri:
The Prophet (sallallaahu alayi wa sallam) 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 Messenger (sallallaahu alayhi wa sallam)?" Allah Apostle replied: " Allah (the) One, the Self-Sufficient Master Whom all creatures need.' (Qur'an Suurah al-'ikhlaas 112) is equal to one third of the Qur'an."
Sahiih al-Bukhaari Hadiith No. 5015
ومنهم من حمل المعادلة على تحصيل الثواب فقال: معنى كونها ثلث القرآن أن ثواب قراءتها يحصل للقارئ مثل ثواب من قرأ ثلث القرآن
According to a group of scholars "to equal 1/3 of the Qur'aan" means that the person who recites it will get the reward of the one who recites 1/3 of the Qur'aan.
Volume 3 Page 624