It was narrated from Shu'bah who said: "I said to Hammad: 'I heard Mansur, and Sulaiman, and Zubaid narrating from Abu Wa'il, from 'Abdullah, that the Messenger of Allah (ﷺ) said: "Defaming a Muslim is evildoing and fighting him is Kufr." - Who are you worried about? Are you worried about Mansur? Are you worried about Zubaid? Are you worried about Sulaiman?' He said: 'No, but I am worried about Abu Wa'il.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا أَبُو دَاوُدَ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ قُلْتُ لِحَمَّادٍ سَمِعْتُ مَنْصُورًا، وَسُلَيْمَانَ، وَزُبَيْدًا، يُحَدِّثُونَ عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " . مَنْ تَتَّهِمُ أَتَتَّهِمُ مَنْصُورًا أَتَتَّهِمُ زُبَيْدًا أَتَتَّهِمُ سُلَيْمَانَ قَالَ لاَ وَلَكِنِّي أَتَّهِمُ أَبَا وَائِلٍ .
sunnah.com