Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1772
Abu Huraira reported: The Messenger of Allah (may peace be upon him) 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.