كتاب السلام
6
The Book of Greetings
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 صلى الله عليه وسلم‏:‏ ‏"‏إذا سلم عليكم أهل الكتاب فقولوا‏:‏ وعليكم” ‏(‏‏(‏متفق عليه‏)‏‏)‏
Sunnah.com reference : Book 6, Hadith 23
Arabic/English book reference : Book 6, Hadith 867