سِبابُ المؤمِنِ فُسوقٌ، وقِتالُهُ كُفرٌ، وأكلُ لَحمِهِ مِن مَعصيَةِ اللَّهِ.

To insult a believer is a gross iniquity, to kill him is infidelity, and to backbite him is an act of disobedience of Allah.

Bihar al-Anwar, v. 75, p. 148, no. 6