Musnad Ahmad 2716

It was narrated from Ibn 'Abbas (رضي الله عنه) that An Ansari man insulted one of the forefathers of al-'Abbas who lived at the time of Jahiliyyah, and al-'Abbas slapped him. His people came and said: By Allah we will certainly slap him (al-'Abbas) as he slapped him (our man), and they took up arms. News of that reached the Messenger of Allah (ﷺ), so he ascended the minbar and said: "O people, who among the people of this world is dearest to Allah?" They said: You are. He said: "al-'Abbas is of me and I am of him, so do not revile our dead lest you offend our living.” The people came and said: O Messenger of Allah, we seek refuge with Allah from your anger.