Al-Kāfi - Volume 1 > Chapter on the necessity to seek knowledge and the recommendations to learn
Hadith #7

7ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ عَنِ الْقَاسِمِ بْنِ الرَّبِيعِ عَنْ مُفَضَّلِ بْنِ عُمَرَ قَالَ سَمِعْتُ أَبَا عَبْدِ الله (عَلَيْهِ السَّلام) يَقُولُ عَلَيْكُمْ بِالتَّفَقُّهِ فِي دِينِ الله وَلا تَكُونُوا أَعْرَاباً فَإِنَّهُ مَنْ لَمْ يَتَفَقَّهْ فِي دِينِ الله لَمْ يَنْظُرِ الله إِلَيْهِ يَوْمَ الْقِيَامَةِ وَلَمْ يُزَكِّ لَهُ عَمَلاً.

7. Al-Hassan ibn Muhammad has narrated from Ja‘far ibn Muhammad from al-Qasim ibn al-Rabi‘ from Mufaddal ibn ‘Umar who has said the following. “I heard abu ‘Abdallah (a.s.) say, ‘It is necessary for you to have good understanding of religion of Allah. Do not be like Bedouins because those who do not gain good understanding of religion, Allah will not look to then on the Day of Judgement and none of his deeds will be cleansed.’”