A Jew passed by the Messenger of Allah and said: As-Sam Aleikum (Death upon you) [As-Sam is a homophone of Arabic word As-Salam that means death). So, the Messenger of Allah replied: And upon you. Then he asked his companions: Do you know what he has said? Death upon you! The companions said: Shall we not kill him? The Prophet (PBUH) said: No, but when the People of the Scriptures greet you, then say: And upon you.

When a Jew greets you, he is only saying: As-Samu Aleikum (Death upon you) [As-Sam is a homophone of Arabic word As-Salam that means death), so reply: Alaykum (upon you).