(202) بَابُ سِبَابِ الْمُسْلِمِ فُسُوقٌ
Sa'id ibn Malik reported that the Prophet, may Allah bless himand grant him peace, said, "Reviling a Muslim is deviant behaviour."
Reference: Al-Adab Al-Mufrad 429
In-book reference: Book 24, Hadith 11
English translation: Book 24, Hadith 429