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]. (DE)
App reference (DE): Book 6, Hadith 23 (DE)
Arabic/English book reference (DE): Book 6, Hadith 867 (DE)