Musnad Ahmad 32:1250

Sulayman ibn Dawud narrated to us, Shu'bah narrated to us, from Hisham ibn Zayd ibn Anas, who said: I heard Anas saying: A man from the People of the Scripture came and greeted the Prophet, may Allah bless him and grant him peace, and said: "As-samu 'alaykum" (Death be upon you). Umar said: "O Messenger of Allah, shall I not strike his neck?" He said: "No. If they greet you, then say: 'Wa 'alaykum' (And upon you)."