It is narrated from Sayyiduna Iyad bin Himar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whatever two people who insult each other say, its sin is upon the one who started it, as long as the wronged one does not exceed the limit."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15074