Sahih Al Bukhari 1: Chapter 3, Hadith 127

(127) Whoever selected some people to teach them (religious) knowledge preferring them over others for fear that the others may not understand it

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، ‏‏‏‏‏‏عَنْ مَعْرُوفِ بْنِ خَرَّبُوذٍ، ‏‏‏‏‏‏عَنْ أَبِي الطُّفَيْلِ، ‏‏‏‏‏‏عَنْ عَلِيٍّ بِذَلِكَ.

SAHIH HADITH:

Narrated Abu at-Tufail: The above mentioned Statement of `Ali.

REFERENCE:

Sahih Al Bukhari 1: Chapter 3, Hadith 127