(202) بَابُ سِبَابِ الْمُسْلِمِ فُسُوقٌ
'Abdullah reported that the Prophet, may Allah bless him and granthim peace, said, "Reviling a Muslim is deviant behaviour and killing himis disbelief."
Reference: Al-Adab Al-Mufrad 431
In-book reference: Book 24, Hadith 13
English translation: Book 24, Hadith 431