It is narrated from Sayyiduna Abdullah bin Mughaffal (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Abusing a Muslim is wickedness (fusuq), and fighting him is disbelief (kufr)."
No one narrated this hadith from Salih except Maymun, and Kathir bin Yahya is alone in narrating it from him.
Hadith Referenceالمعجم الأوسط / باب الألف / 734
Hadith Gradingمحدثین:إسناده ضعيف لضعف كثير بن يحيى
Hadith Takhrij«قال الهيثمي: فيه كثير بن يحيى وهو ضعيف «مجمع الزوائد ومنبع الفوائد، ج 8، ص 73»»