Hadith 4818
وَعَن أنس وَأَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «الْمُسْتَبَّانِ مَا قَالَا فَعَلَى الْبَادِئِ مالم يعْتد الْمَظْلُوم» . رَوَاهُ مُسلم
It is narrated from Anas and Abu Huraira (may Allah be pleased with them) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Of two people who abuse each other, the sinner is the one who starts it, as long as the oppressed does not retaliate with more than one insult for one." Narrated by Muslim.