He traced it back to the Prophet ﷺ, who said, "You will find that among the worst of people is the two-faced one, who comes to these with one face and to those with another."
حدثنا سفيان عن أبي الزناد عن الأعرج
يبلغ به النبي ﷺ قال تجدون من شر الناس ذا الوجهين الذي يأتي هؤلاء بوجه وهؤلاء بوجه
Its isnad is sahih. Abu Dawud narrated it, 4872 [4:419 Awn al-Mabud], from Musaddad, from Sufyan, with this wording. Malik narrated it in Muwatta, 991, from Abu Zinad. Muslim narrated it, 2:288, by way of Malik. Bukhari narrated it, 6:384–385, at length within a hadith, and 10:395, 13:150, and Muslim also narrated it, and Tirmidhi, 3:153, from other routes.