Authentic hadithNarrated by Bukhari & Muslim
Reviling a Muslim is Fusūq (deobedience) and fighting him is Kufr (disbelief)
عَنْ عَبْدِ اللَّهِ بنِ مَسْعُودٍ رضي الله عنه أَنَّ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «سِبَابُ المُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ».
Explanation
The Prophet (may Allah's peace and blessings be upon him) forbade the Muslim from cursing and reviling his Muslim brother, stating that this is an act of Fusūq, which means disobeying Allah and His Messenger (may Allah's peace and blessings be upon him), and fighting his Muslim brother is an act of Kufr, yet it is minor Kufr.
Source
صحيح البخاري (1/ 19) (48).
صحيح مسلم (1/ 81) (64).
فتح ذي الجلال والاكرام بشرح بلوغ المرام، لابن عثيمين (6/ 362).
منحة العلام في شرح بلوغ المرام، لعبد الله الفوزان (10/ 216).
توضِيحُ الأحكَامِ مِن بُلوُغ المَرَام، للبسام (7/ 412).
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