Musnad Ahmad 626:77

'Abd al-Rahman narrated to us, Sufyan narrated to us, from Ziyad ibn 'Alaqah, who said: I heard a man in the presence of al-Mughirah ibn Shu'bah say: The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not revile the dead, for you will hurt the living."