Sahih Muslim Book 1, Hadith Number 122.

Chapter : Abusing a Muslim is an outrage and fighting against him is unbelief.

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.