It is narrated from Sayyiduna Amr bin al-Nu'man bin Muqarrin (may Allah be pleased with him): The Prophet (peace and blessings be upon him) came to one of the gatherings of the Ansar, among whom was a man well known for foul speech. The Prophet (peace and blessings be upon him) said: "Abusing a Muslim is wickedness (fusuq), and fighting him is disbelief (kufr)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14152