سنن النسائي

It was narrated from Sufyan bin Zubaid, from Abu Wa'il, from 'Abdullah: "The Messenger of Allah (ﷺ) [SAW] said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'" I said to Abu Wa'il: "Did you hear it from 'Abdullah?" He said: "Yes."