Musnad Ahmad 30:3572

Rawh narrated to us, Shu'bah narrated to us, he said: I heard Al-'Ala' ibn 'Abd al-Rahman ibn Ya'qub, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, that he said: "The two who insult one another, what they say is upon the one who began, unless the oppressed transgresses."