It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When a Jew greets you with Salam, he says «السام عليكم», so in reply to him, you should only say «وعليك»."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5938