Language:
كتاب العلم
3
Knowledge
(49)
Chapter: Whoever selected some people to teach them (religious) knowledge preferring them over others for fear that the others may not understand it
(49)
باب مَنْ خَصَّ بِالْعِلْمِ قَوْمًا دُونَ قَوْمٍ كَرَاهِيَةَ أَنْ لاَ يَفْهَمُوا
Sahih al-Bukhari 127

Narrated Abu at-Tufail:

The above mentioned Statement of `Ali.

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى عَنْ مَعْرُوفِ بْنِ خَرَّبُوذٍ عَنْ أَبِي الطُّفَيْلِ عَنْ عَلِيٍّ بِذَلِكَ‏.‏
Reference : Sahih al-Bukhari 127
In-book reference : Book 3, Hadith 69
USC-MSA web (English) reference : Vol. 1, Book 3, Hadith 129
  (deprecated numbering scheme)