From Sahih Bukhari (Hadith 5035):
Narrated Sa`id bin Jubair:Those Suras which you people call the Mufassal, are the Muhkam. And Ibn `Abbas said, "Allah's Apostle died when I was a boy of ten years, and I had learnt the Muhkam (of the Qur'an)
Reference: Bukhari 5035