Al-Kāfi - Volume 2 > Tyrannical Transgression
Hadith #3

3ـ عَلِيٌّ عَنْ أَبِيهِ عَنْ حَمَّادٍ عَنْ حَرِيزٍ عَنْ مِسْمَعٍ أَبِي سَيَّارٍ أَنَّ أَبَا عَبْدِ الله (عَلَيهِ السَّلام) كَتَبَ إِلَيْهِ فِي كِتَابٍ انْظُرْ أَنْ لا تُكَلِّمَنَّ بِكَلِمَةِ بَغْيٍ أَبَداً وَإِنْ أَعْجَبَتْكَ نَفْسَكَ وَعَشِيرَتَكَ.

3. Ali has narrated from his father from Hammad from Hariz from Misma’ abu Sayyar who has said the following: “Abu ‘Abd Allah (a.s.) once wrote a letter to me in which he had said, ‘Keep in mind not to ever speak a tyrannical and transgressing word even though your own soul and tribe are more captivating to you.’”