Riyad us Saliheen: Hadith 867

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].