Sahih al-Bukhari 48
Sahih al-Bukhari · Book 2: Belief · (What is said regarding) the fear of a believer that his good deeds may be annulled (lost) without his knowledge
Narrated 'Abdullah:
حَدَّثَنَا مُحَمَّدُ بْنُ عَرْعَرَةَ ، قَالَ حَدَّثَنَا شُعْبَةُ ، عَنْ زُبَيْدٍ ، قَالَ سَأَلْتُ أَبَا وَائِلٍ عَنِ الْمُرْجِئَةِ،، فَقَالَ حَدَّثَنِي عَبْدُ اللَّهِ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " سِبَابُ الْمُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ " .
The Prophet (ﷺ) said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)."