Malik related to me from Abu'z-Zinad from al-Araj from AbuHurayra that the Messenger of Allah, may Allah bless him and grant himpeace, said, "One of the most evil of people is the two-faced personwho shows one face to these people and another face to those people."
USC-MSA web (English) reference: Book 56, Hadith 21
Arabic reference: Book 56, Hadith 1834