Sahih Al Bukhari 1: Chapter 2, Hadith 48

(48) (What is said regarding) the fear of a believer that his good deeds may be annulled (lost) without his knowledge

حَدَّثَنَا مُحَمَّدُ بْنُ عَرْعَرَةَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا شُعْبَةُ، ‏‏‏‏‏‏عَنْ زُبَيْدٍ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ سَأَلْتُ أَبَا وَائِلٍ عَنِ الْمُرْجِئَةِ، ‏‏‏‏‏‏فَقَالَ، ‏‏‏‏‏‏حَدَّثَنِي عَبْدُ اللَّهِ، ‏‏‏‏‏‏أَنّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ "سِبَابُ الْمُسْلِمِ فُسُوقٌ، ‏‏‏‏‏‏وَقِتَالُهُ كُفْرٌ".

SAHIH HADITH:

Narrated 'Abdullah: The Prophet said, Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief).

REFERENCE:

Sahih Al Bukhari 1: Chapter 2, Hadith 48