Musnad Ahmad 32:1515

'Abd al-Wahhab narrated to us, from Sa'id, from Qatadah, from Anas ibn Malik, that a Jew passed by the Messenger of Allah, peace and blessings of Allah be upon him, and his companions and said: "As-samu 'alaykum" (Death be upon you). The Prophet of Allah, peace and blessings of Allah be upon him, said: "Do you know what he said?" They said: "He greeted you, O Messenger of Allah." He said: "No, rather he said such and such." Then he said: "Bring him back to me." So they brought him back to him. He said: "Did you say: 'As-samu 'alaykum'?" He said: "Yes." The Prophet of Allah, peace and blessings of Allah be upon him, then said: "If one of the People of the Book greets you, then say: 'Wa 'alayk' (And upon you)," meaning: And upon you be what you said.