Musnad Ahmad 32:487

Muhammad ibn Bishr narrated to us, Sa'id ibn Abi 'Arubah narrated to us, from Qatadah, from Anas ibn Malik, that a Jew greeted the Messenger of Allah, peace and blessings of Allah be upon him, and said: "As-sam 'alayk" (Death be upon you). He said: "Bring him back to me." He said: "Did you say: 'As-sam 'alayk'?" He said: "Yes." The Messenger of Allah, peace and blessings of Allah be upon him, said: "If one of the People of the Book greets you, then say: 'Wa 'alayk' (And upon you)."