Musnad Ahmad 489:6

Yazid ibn Harun narrated to us, Hammam informed us, from Qatadah, from Yazid ibn Abdullah, from Iyad ibn Himar, from the Prophet (peace and blessings of Allah be upon him): "The sin of the two who revile each other is upon the one who began, unless the oppressed person transgresses." Or he said: "Until the oppressed person transgresses." (Yazid was in doubt).