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.