Musnad Abu Ya'la al-Mawsili 160:86

Abu Khaythama narrated to us, Ismail narrated to us, from Rawh ibn al-Qasim, 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 those who insult one another, whatever they say is upon the one who initiates it, as long as the oppressed does not transgress."