The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1770
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.