From Sahih Bukhari (Hadith 6257):
Narrated `Abdullah bin `Umar:Allah's Messenger (ﷺ) said, "When the Jews greet you, they usually say, 'As-Samu 'alaikum (Death be on you),' so you should say (in reply to them), 'Wa'alaikum (And on you)
Reference: Bukhari 6257