Sahih Muslim Book 4, Hadith Number 1772.

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

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.