Musnad Ahmad 32:1297

Abdullah ibn Bakr narrated to us, he said: Sa’id informed us, from Qatadah, from Anas, that a Jew came to the Prophet, may Allah bless him and grant him peace, while he was with his companions, and said: "As-samu 'alaykum" (Death be upon you). The people responded to him. The Prophet of Allah, may Allah bless him and grant him peace, said: "Do you know what he said?" They said: "Yes, he said: 'As-samu 'alaykum'." He said: "Bring the man back to me." So they brought him back. The Prophet of Allah, may Allah bless him and grant him peace, said: "Did you say such and such?" He said: "Yes." The Prophet of Allah, may Allah bless him and grant him peace, said: "If one of the People of the Book greets you, then say: 'Wa 'alaykum' (And upon you)," meaning: upon you is what you said.