The Prophet ﷺ said, "Among the worst of people is the two-faced person, who comes to one group with one face and to another group with a different face."
حدثنا مسدد حدثنا سفيان عن أبي الزناد عن الأعرج
أن النبي ﷺ قال من شر الناس ذو الوجهين الذي يأتي هؤلاء بوجه وهؤلاء بوجه
Its isnad is sahih.Ahmad narrated it in 2/245, and Humaydi in his Musnad, no. 1139 (in my edition), from Sufyan bin Uyayna, through this route, and he stated explicitly that he had heard it directly. Muslim also narrated it, no. 2526, from the hadith of Abu Zinad, through this route.