Musnad Ahmad 32:8

Hushaym narrated to us, Ubayd Allah ibn Abi Bakr ibn Anas informed us, from his grandfather Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If the People of the Book greet you, then say: 'Wa 'alaykum' (And upon you)."