It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Abusing a Muslim is wickedness, and fighting him is disbelief." The narrator says: I asked Abu Wa'il, "Did you hear this from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) yourself?" He replied: "Yes!"
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 3903