Narrated 'Abdullah:

The Prophet (s.a.w.a.) said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)."

It is narrated on the authority of 'Abdullah b. Mas'ud that the Messenger of Allah (s.a.w.a.) 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 (s.a.w.a.)? He replied: Yes. But there is mention of the talk between Zubaid and Abu Wa'il in the hadith narrated by Shu'ba.