When the Jews offer you salutations, some of them say as-Sam-u-'Alaikum (death be upon you). You should say (in response to it): Let it be upon you.
Sahih Muslim Book 39, Hadith 9
← Previous Hadith The People. of the Book offer us salutations (by saying as-Salamu- 'Alaikum). How should we reciprocate them? Thereupon he said: Say: Wa 'Alaikum (and upon you too).

Sahih Muslim Book 39, Hadith 8 Next Hadith → This hadith has been narrated on the authority of Ibn 'Umar through another chain of transmitters with a slight variation of wording.

Sahih Muslim Book 39, Hadith 10