Musnad Ahmad 27:251

Yahya narrated to us, from Sufyan, Abdullah ibn Dinar narrated to me, he said: I heard Ibn Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Jews greet you, they only say: 'As-samu 'alayk' (Death be upon you), so say: 'Wa 'alayk' (And upon you)."