Ziyad bin Ilaqah narrates that a man spoke ill of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), so Sayyiduna Zayd bin Arqam (may Allah be pleased with him) said to him: "Do you not know that the Messenger of Allah (peace and blessings be upon him) used to forbid speaking ill of the dead?"
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4956