Sunan an-Nasa'i 4104
سنن النسائي#4104SahihSa'd bin Abi Waqas told us that:The Messenger of Allah [SAW] said: "Fighting a Muslim is Kufr and defaming him is evildoing
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 37:139