Sunan An-Nasa'i

It was narrated from Shu'bah, from Abu Ishaq who said:""I heard Abu Al-Ahwas (narrate) that 'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'"" So Aban said to him: ""O Abu Ishaq! You heard it only from Abu Al-Ahwas?"" He said: ""Rather, I heard it from Al-Aswad and Hubairah