The Prophet ﷺ said, "When the People of the Book greet you with peace, respond by saying, 'And upon you.'"In another wording, it was asked, "The People of the Book greet us with peace. How should we return their greeting?"He replied, "Say, 'And upon you.'"
قال النبي ﷺ: إذا سلم عليكم أهل الكتاب فقولوا: وعليكم. وفي لفظ: إن أهل الكتاب يسلمون علينا فكيف نرد عليهم؟ قال: قولوا: وعليكم.
Bukhari reported it in the Book of Seeking Permission (6258), and Muslim in the Book of Greetings (6:2163), through various 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 reported it (2163:7) through various routes from Shubah, from Qatada, from Anas, and he related it with the second wording.