Sayyiduna Anas (may Allah be pleased with him) narrates: A Jew greeted the Prophet Muhammad (peace be upon him) and his companions by saying "As-Saam Alaikum" (death be upon you). The Prophet (peace be upon him) asked: "Do you know what he said?" The people replied: "Yes! He greeted us with Salam." The Prophet (peace be upon him) said: "No. He said 'As-Saam Alaikum,' meaning your religion is weak." So when any person from the People of the Book greets you (in this way), then you should say, 'Wa Alaik' (and the same be upon you), and the same be upon you."