Anas (May Allah be pleased with him) reported:
Messenger of Allah ﷺ said, "When the people of the Book greet you (i.e., by saying 'As-Samu 'Alaikum,' meaning death be upon you), you should respond with: 'Wa 'alaikum' [The same on you (i.e., and death will be upon you, for no one will escape death)]."

[Al-Bukhari and Muslim].

وعن أنس رضي الله عنه قال: قال رسول الله ﷺ: "إذا سلم عليكم أهل الكتاب فقولوا: وعليكم” ((متفق عليه))