Sahih al-Bukhari 7076
صحيح البخاري#7076SahihNarrated `Abdullah:The Prophet, said, "Abusing a Muslim is Fusuq (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: 92:27