Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6300
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: The worst amongst the people is the double-faced one; he comes to some people with one face and to others with the other face.