The paternal uncle of Ziyad bin Ilaqah narrates that Mughirah bin Shu'bah (may Allah be pleased with him) (said something improper about Sayyiduna Ali (may Allah be pleased with him)), so Sayyiduna Zayd bin Arqam (may Allah be pleased with him) stood up towards him and said: "O Mughirah! Do you not know that the Messenger of Allah (peace and blessings be upon him) forbade speaking ill of the dead? So why do you speak ill of Ali when he has passed away?"
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4955