Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that a man, during the Jahiliyyah, insulted one of the fathers of Sayyiduna al-Abbas (may Allah be pleased with him) (i.e. one of his forefathers), so Sayyiduna al-Abbas (may Allah be pleased with him) slapped him. His people came and said: "By Allah, we will certainly slap him (al-Abbas)," and they took up arms. News of this reached the Messenger of Allah (peace and blessings be upon him), so he (peace and blessings be upon him) ascended the pulpit and said: "O people! Whom among the people of the earth do you know to be the most honoured before Allah?" The people said: "You." He (peace and blessings be upon him) said: "Indeed, al-Abbas is from me and I am from him. Do not revile our dead, for that would hurt our living." So those people came and said: "O Messenger of Allah! We seek refuge with Allah from your anger. Ask forgiveness for us."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12342