Musnad Abu Ya'la al-Mawsili 148:9

Abu Ali al-Hasan, a companion of ours, narrated to us, Yunus narrated to us, Layth narrated to us, from Yazid ibn Abi Habib, from Ibn Sinan, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Regarding two people who insult one another, whatever they say is upon the one who initiated it, unless the oppressed person transgresses."