From Sahih Bukhari (Hadith 5036):
Narrated Sa`id bin Jubair:Ibn `Abbas said, "I have learnt all the Muhkam Suras during the life time of Allah's Messenger (ﷺ)." I said to him, 'What is meant by the Muhkam?" He replied, "The Mufassal
Reference: Bukhari 5036