Musnad Ahmad 30:3297

Ibn Numayr narrated to us, from al-A‘mash, and Ya‘la said: Al-A‘mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "You will find the worst of people—and Ya‘la said: You will find among the worst of people—before Allah on the Day of Resurrection to be the two-faced person." Ibn Numayr said: The one who comes to these with the speech of these, and to these with the speech of these.