It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Abusing a Muslim is sinfulness (fusuq), and fighting him is disbelief (kufr)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10270