The Prophet ﷺ said, "When the People of the Book greet you with salaam, respond by saying, 'And upon you.'"In another wording recorded by Muslim, the companions of the Prophet ﷺ said to him, "The People of the Book greet us with salaam. How should we respond to them?"He replied, "Say, 'And upon you.'"
قال النبي ﷺ: إذا سلم عليكم أهل الكتاب فقولوا: وعليكم. وفي لفظ لمسلم: إن أصحاب النبي ﷺ قالوا للنبي: إن أهل الكتاب يسلمون علينا فكيف نرد عليهم؟ قال: قولوا: وعليكم.
Bukhari narrated it in the Book of Seeking Permission (6258), and Muslim in the Book of Greetings (2163: 6), through multiple routes from Hushaym, [who said], Ubaydullah bin Abu Bakr bin Anas informed us, from his grandfather Anas bin Malik, and he related it.Muslim also narrated it (2163: 7) through multiple routes from Shubah, from Qatada, from Anas, with the second wording.