Musnad Ahmad 489:8

Bahz and ‘Affan narrated to us, they said: Hammam narrated to us. ‘Affan said in his narration: Qatadah narrated to us, from Yazid, the brother of Mutarrif, from ‘Iyad ibn Himar, that the Prophet (peace and blessings of Allah be upon him) said: "The sin of the two who revile each other is upon the one who began, as long as the oppressed does not transgress." ‘Affan said: Or, "until the oppressed transgresses."