The Messenger of Allah ﷺ said, "He is not one of us who imitates other than us. Do not imitate the Jews or the Christians, for the greeting of the Jews is by gesturing with the fingers, and the greeting of the Christians is by gesturing with the palms."
حدثنا قتيبة حدثنا ابن لهيعة
أن رسول الله ﷺ قال ليس منا من تشبه بغيرنا لا تشبهوا باليهود ولا بالنصارى فإن تسليم اليهود الإشارة بالأصابع وتسليم النصارى الإشارة بالأكف
No references are available.
Its isnad is daif. Ibn Jawzi related it in Ilal Mutanahiya (2/234, no. 1201) through the route of Tirmidhi, with the same wording. Ibn Lahia is a mudallis and he narrated it using "an" (indicating possible concealment in the chain). The hadith has daif corroborating narrations found in Tabarani's Awsat (8/184, 185, no. 3760) and in Nasai's Kubra (no. 10172), and others.