Translation
(66) Virtues of the Qur'an
كتاب فضائل القرآن
(25) Chapter: Teaching the Qur'an to the children
(25) باب تَعْلِيمِ الصِّبْيَانِ الْقُرْآنَ
Sahih al-Bukhari 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).

حَدَّثَنِي مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي بِشْرٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، قَالَ إِنَّ الَّذِي تَدْعُونَهُ الْمُفَصَّلَ هُوَ الْمُحْكَمُ، قَالَ وَقَالَ ابْنُ عَبَّاسٍ تُوُفِّيَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَأَنَا ابْنُ عَشْرِ سِنِينَ وَقَدْ قَرَأْتُ الْمُحْكَمَ‏.‏
Reference : Sahih al-Bukhari 5035
In-book reference : Book 66, Hadith 58
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 554
  (deprecated numbering scheme)