Musnad Ahmad 27:116

Sufyan narrated to us, I heard it from Ibn Dinar, from Ibn Umar, from the Prophet, peace and blessings of Allah be upon him: "If a Jew greets you, he is only saying: 'As-sam (death/calamity) be upon you,' so say: 'Wa 'alayk (and upon you).'" And he said another time: "If the Jews greet you, then say: 'Wa 'alaykum (and upon you),' for they say: 'As-sam (death/calamity) be upon you.'"