It is narrated from Hazrat Anas (may Allah be pleased with him) that a Jew greeted the Prophet (peace and blessings be upon him) and the companions (may Allah be pleased with them) by saying "As-Saam Alaikum" (death be upon you). The Prophet (peace and blessings be upon him) said to the companions (may Allah be pleased with them), "He said 'As-Saam Alaikum'." The Jew was brought and he admitted it. The Prophet (peace and blessings be upon him) said, "When a person of the Book greets you, reply to him with the same phrase."