It is narrated on the authority of 'Abdullah b. Mas'ud that the Messenger of Allah (ﷺ) 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 (ﷺ)? He replied: Yes. But there is mention of the talk between Zubaid and Abu Wa'il in the hadith narrated by Shu'ba.
Reference: Sahih Muslim 64 a In-book : Book 1, Hadith 127 USC-MSA web (English) : Book 1, Hadith 122 (deprecated)
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