Riyad as-Salihin, Book 6, Hadith 867 (In-Book) ⓘ
Anas (May Allah be pleased with him) reported: Messenger of Allah 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 صلى الله عليه وسلم: "إذا سلم عليكم أهل الكتاب فقولوا: وعليكم” ((متفق عليه))