Musnad Abu Ya'la al-Mawsili 115:75

Abu Bakr ibn Abi Shaybah narrated to us, 'Abdah narrated to us, from Sa'id ibn Abi 'Arubah, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If the People of the Scripture greet you, then say: 'Wa 'alaykum' (And upon you)."