رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 1561
The Book of the Prohibited actions
Prohibition of Reviling a Muslim without any cause
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّه عَلَيْهِ وَسَلَّمَ قَالَ: المتسابان مَا قَالاَ فَعَلَى الْبَادِي مِنْهُمَا حتى يعتدي المظلوم ((رَوَاهُ مُسْلِم)).
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].
Riyad as-Salihin, 1561