Sunan an-Nasa'i 4108
سنن النسائي#4108SahihIt was narrated from 'Abdur-Rahman bin 'Abdullah, from his father, that:The Messenger of Allah [SAW] said: "Defaming a Muslim is evildoing and fighting him is Kufr
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih Isnaad
Al-AlbaniSahih Isnaad
Zubair Ali ZaiSahih
In-book reference: 37:143