From Sahih Muslim (Hadith 64):
It is narrated on the authority of 'Abdullah b. Mas'ud that the Messenger of Allah (may peace and blessings be upon him) observed: Abusing a Muslim is an outrage and fighting against him is unbelief. Zubaid said: I asked Abu Wa'il: Did you hear it from Abdullah narrating if from the Messenger of Allah (may peace and blessings be upon him)? He replied: Yes. But there is mention of the talk between Zubaid and Abu Wa'il in the hadith narrated by Shu'ba.
Reference: Muslim 64