The Messenger of Allah ﷺ said, "Reviling a Muslim is grave disobedience, and fighting him is disbelief."Whom do you suspect? Do you suspect Mansur? Do you suspect Zubayd? Do you suspect Sulayman?He replied, "No, but I suspect Abu Waail."
أخبرنا محمود بن غيلان قال حدثنا أبو داود قال حدثنا شعبة قال قلت لحماد سمعت منصورا وسليمان وزبيدا يحدثون عن أبي وائل
أن رسول الله ﷺ قال سباب المسلم فسوق وقتاله كفر من تتهم أتتهم منصورا أتتهم زبيدا أتتهم سليمان قال لا ولكني أتهم أبا وائل
Bukhari recorded it in Kitab al-Iman, chapter on the believer's fear that his deeds might be nullified without his realizing it, hadith 48, and Muslim in Kitab al-Iman, chapter explaining the Prophet's ﷺ statement, "Cursing a Muslim is transgression and fighting him is disbelief," hadith 64, from the hadith of Shubah with this same chain. It is also found in al-Kubra, hadith 3574.Hammad here is Ibn Abu Sulayman, who was a Murjii, one of the people of innovation, and his hadith is hasan.