Musnad Ahmad 30:3199

Isma'il narrated to us, he said: Rawh ibn al-Qasim informed us, from al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub, from his father, from Abu Hurayrah, who said: 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, as long as the oppressed does not transgress."