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 twice: Did you hear this directly from Sayyiduna Ibn Mas'ud (may Allah be pleased with him)? He replied: Yes!
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4345