Sahih Muslim Book 4, Hadith Number 1770.

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

This Hadith has been narrated by Qatada with the same chain of transmitters in these words: He (the Apostle of Allah) said: Allah divided the Qur’an into three parts, and he made: “Say: He, Allah is One.” one part out of the (three) parts of the Qur’an.