It is narrated from Sayyiduna Iyad bin Himar (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whatever two people who insult each other say, its sin is upon the one who started it, unless the wronged one exceeds the limit."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15073