Sahih Bukhari
Virtues of the Quran
Volume: 006 / Book Number: 061 / Hadith Number: 555
Narrated Said bin Jubair:
Ibn 'Abbas said, "I have learnt all the Muhkam Suras during the life time of Allah's Apostle." I said to him, 'What is meant by the Muhkam?" He replied, "The Mufassal."