Ibn Mas'ud (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Abusing a Muslim is a sin, and killing him, fighting him, is disbelief." Zubayd said: I asked Abu Wa'il: Did you hear this hadith as marfu' (from the Prophet)? He said: Yes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15852