Daily Hadith Wed 11-Jul-2018 - 28 Shawwal 1439 حديث اليوم
باب تحريم سَبّ المسلم بغير حقّ
وعنِ ابنِ مَسعودٍ رضي اللَّه عَنهُ قال : قال رسُولُ اللَّه صَلّى اللهُ عَلَيْهِ وسَلَّم : « سِباب المُسْلِمِ فُسوقٌ ، وقِتَالُهُ كُفْرٌ » متفقٌ عليه .
1559/266 - رياض الصالحين
Prohibition of Reviling a Muslim without any cause
Ibn Mas`ud (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "Reviling a Muslim is Fusuq (disobedience of Allah) and killing him is (tantamount to) disbelief."[Al-Bukhari and Muslim].

"Killing him is (tantamount) to disbelief" means that in terms of sin and prohibition, it is akin to Kufr. This Hadith makes the severity of this crime abundantly clear.

266/1559 - Riyad Us-Saliheen (Gardens of the Righteous)
Wed 11-Jul-2018 - 28 Shawwal 1439