Reviling a Muslim is deviant behaviour Al-Adab Al-Mufrad 433
Arabic hadith text
وَبِالسَّنَدِ، عَنْ أَبِي ذَرٍّ، سَمِعَ النَّبِيَّ صلى الله عليه وسلم يَقُولُ: مَنِ ادَّعَى لِغَيْرِ أَبِيهِ وَهُوَ يَعْلَمُ فَقَدْ كَفَرَ، وَمَنِ ادَّعَى قَوْمًا لَيْسَ هُوَ مِنْهُمْ فَلْيَتَبَوَّأْ مَقْعَدَهُ مِنَ النَّارِ، وَمَنْ دَعَا رَجُلاً بِالْكُفْرِ، أَوْ قَالَ: عَدُوُّ اللهِ، وَلَيْسَ كَذَلِكَ إِلا حَارَتْ عَلَيْهِ.
Translation
Abu Dharr states that he heard the Prophet, may Allah bless him and grant him peace, say, "A person who knowingly claims a father other than his own has disbelieved. A person who claims to be from a people when he is not one of them will take his place in the Fire. A person who calls a man an unbeliever or says, 'Enemy of Allah,' when that is not the case will have that come back on him."