Abu Hurayra reported that the Messenger of Allah, may Allah blesshim and grant him peace, said, "The worst person is someone who is two-faced.He is the person who comes to one group of people with one face and anothergroup of people with another face." (ID)
English reference (ID): Book 56, Hadith 1309 (ID)
Arabic reference (ID): Book 1, Hadith 1309 (ID)