It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a man mentioned one of the (ancestral) elders of Sayyiduna Abbas (may Allah be pleased with him) disparagingly, whereupon Sayyiduna Abbas (may Allah be pleased with him) slapped him. Then that man's tribesmen gathered and said: By Allah! He too will slap Abbas just as he slapped him. This reached the Messenger of Allah (peace and blessings be upon him), so he (peace and blessings be upon him) delivered a sermon and said: "Who among people is the most honoured in the sight of Allah?" The Companions said: O Messenger of Allah! You are. He (peace and blessings be upon him) said: "Indeed, Abbas is from me and I am from him. Do not revile our dead, for by that you would hurt the living." This hadith has an authentic (sahih) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.