Sahih Muslim Book 26, Hadith Number 5382.

Chapter : Prohibition of saying first as-Salam-u-Alaikum to the people of the book, and how their salutations should be responded.

Ibn ‘Umar reported Allah’s Messenger (may peace be upon him) as saying: When the Jews offer you salutations, tome of them say as-Sam-u-‘Alaikum (death be upon you). You should say (in response to it): Let it be upon you.