Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5382
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.