Musnad Ahmad 27:772

Waki’ narrated to us, Sufyan and ‘Abd al-Rahman narrated to us, from Sufyan, from ‘Abdullah ibn Dinar, from Ibn ‘Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Jews meet you, they say: 'As-samu ‘alaykum' (Death be upon you). So say: 'Wa ‘alaykum' (And upon you)."