رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 1559
The Book of the Prohibited actions
Prohibition of Reviling a Muslim without any cause
وَعَنْ اِبْنِ مَسْعُودٍ رَضِيَ اللَّه عَنْهُ قَالَ: قَالَ رَسُولُ اَللَّهِ صَلَّى اللَّه عَلَيْهِ وَسَلَّمَ : سِبَابُ اَلْمُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ ((مُتَّفَقٌ عَلَيْهِ)).
Ibn Mas'ud (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "Reviling a Muslim is Fusuq (disobedience of Allah) and killing him is (tantamount to) disbelief." [Al-Bukhari and Muslim].
Riyad as-Salihin, 1559