Al-Kāfi - Volume 1 > Chapter on Giving Knowledge as Charity
Hadith #3

3ـ وَبِهَذَا الاسْنَادِ عَنْ أَبِيهِ عَنْ أَحْمَدَ بْنِ النَّضْرِ عَنْ عَمْرِو بْنِ شِمْرٍ عَنْ جَابِرٍ عَنْ ابي جعفر (عَلَيْهِ السَّلام) قَالَ زَكَاةُ الْعِلْمِ أَنْ تُعَلِّمَهُ عِبَادَ الله.

3. Through the same chain of narrators he has narrated from his father from Ahmad ibn Nadr from ‘Amr and ibn Shimr from Jabir from abu Ja’far (a.s.) who has said the following. “Zakat, the tax, on knowledge is teaching it to the servants of Allah.”