Sahih Muslim 2587
Sahih Muslim · Book 45: The Book of Virtue, Enjoining Good Manners, and Joining of the Ties of Kinship · The Prohibition Of Reviling
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنَا يَحْيَى بْنُ أَيُّوبَ ، وَقُتَيْبَةُ ، وَابْنُ، حُجْرٍ قَالُوا حَدَّثَنَا إِسْمَاعِيلُ، - يَعْنُونَ ابْنَ جَعْفَرٍ - عَنِ الْعَلاَءِ ، عَنْ أَبِيهِ ، عَنْ أَبِي هُرَيْرَةَ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الْمُسْتَبَّانِ مَا قَالاَ فَعَلَى الْبَادِئِ مَا لَمْ يَعْتَدِ الْمَظْلُومُ " .
When two persons indulge in hurling (abuses) upon one another, it would be the first one who would be the sinner so long as the oppressed does not transgress the limits.