Al-Kāfi - Volume 1 > Chapter on the necessity to seek knowledge and the recommendations to learn
Hadith #2

2ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ مُحَمَّدِ بْنِ الْحُسَيْنِ عَنْ مُحَمَّدِ بْنِ عَبْدِ الله عَنْ عِيسَى بْنِ عَبْدِ الله الْعُمَرِيِّ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلام) قَالَ طَلَبُ الْعِلْمِ فَرِيضَةٌ.

2. Muhammad ibn Yahya has narrated from Muhammad ibn al-Hassan from Muhammad ibn ‘Abdallah from ‘Isa ibn ‘Abdallah al-‘Amri from abu ‘Abdallah (a.s.) who has said the following. “Seeking knowledge is obligatory.”