Sahih Al Bukhari – Book of Good Manners And Form (Al-Adab)
Volumn 008, Book 073, Hadith Number 070.
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Narated By 'Abdullah RA: Allah's Apostle PBUH said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief)."
