(44) باب مَا يُنْهَى مِنَ السِّبَابِ وَاللَّعْنِ
Narrated `Abdullah:
Allah's Messenger (ﷺ) said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief).
Reference: Sahih al-Bukhari 6044
In-book reference: Book 78, Hadith 74
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 73, Hadith 70