Musnad Ahmad 32:1822

Suwayd ibn ‘Amr al-Kalbi narrated to us, Aban narrated to us, from Qatadah, from Anas ibn Malik, who said: While the Messenger of Allah (peace and blessings of Allah be upon him) was sitting among his companions, a Jew passed by them and greeted them. When he had passed, he called him and said: "What did you say?" He said: I said, "Sam ‘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 ‘alaykum' (And upon you), meaning: whatever you have said."