Prohibition on Speaking without Knowledge.
Al-Kāfi|Volume 1|Book 2|Chapter 11
1ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ وَعَبْدِ الله ابْنَيْ مُحَمَّدِ بْنِ عِيسَى عَنْ عَلِيِّ بْنِ الْحَكَمِ عَنْ سَيْفِ بْنِ عَمِيرَةَ عَنْ مُفَضَّلِ بْنِ يَزِيدَ قَالَ قَالَ لِي أَبُو عَبْدِ الله (عَلَيْهِ السَّلام) أَنْهَاكَ عَنْ خَصْلَتَيْنِ فِيهِمَا هَلاكُ الرِّجَالِ أَنْهَاكَ أَنْ تَدِينَ الله بِالْبَاطِلِ وَتُفْتِيَ النَّاسَ بِمَا لا تَعْلَمُ.
1. Muhammad ibn Yahya has narrated from Ahmad and ‘Abdallah (both) sons of Muhammad ibn ‘Isa from Ali ibn al-Hakam from Sayf ibn ‘Umayra from Mufaddal ibn Yazid that Abu ‘Abdillah (al-Sadiq) عليه السلام said: "I would like to warn you about two forms of behavior that lead a man to destruction. I prohibit you from dealing with Allah on the basis of falsehood and from giving fatwa to people without knowledge.”
- Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (1/136)
- Ṣaḥīḥ al-Kāfī, vol. 1, p. 5