Sahih al-Bukhari 6044
صحيح البخاري#6044SahihNarrated `Abdullah:Allah's Messenger (ﷺ) said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 78:74