Sunan Ibn Majah, Book 36, Hadith 16 (In-Book) ⓘ
It was narrated from Sa’d that the Messenger of Allah said: “Verbally abusing a Muslim is Fusuq (disobedience) and fighting him is Kufr.”
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، عَنْ شَرِيكٍ، عَنْ أَبِي إِسْحَاقَ، عَنْ مُحَمَّدِ بْنِ سَعْدٍ، عَنْ سَعْدٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
also referenced under: Hadith 3941 (Reference) & Volume 5, Book 36, Hadith 3941 (DarusSalam)