Prohibition of Reviling a Muslim without any cause Riyad as-Salihin 1561
Arabic hadith text
وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: "المتسابان ما قالا فعلى البادي منهما حتى يعتدي المظلوم" ((رواه مسلم)).
Translation
Abu Hurairah (May Allah be pleased with him) said
The Messenger of Allah (ﷺ) said, "When two persons indulge in abusing each other, the beginner will be the sinner so long as the oppressed does not transgress the limits."[Muslim].