Musnad Ahmad 32:527

Yunus narrated to us, Aban narrated to us, from Qatadah, from Anas ibn Malik, who said: While the Prophet of Allah, peace and blessings of Allah be upon him, was sitting among his companions, a Jew passed by them and greeted them. The Prophet, peace and blessings of Allah be upon him, said: "Return [the greeting] to him." He said: "How?" I said: "He said: 'Samu 'alaykum' (Death be upon you)." 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) [meaning: whatever you have said]."