Musnad Ahmad 32:1268

'Abd al-Samad narrated to us, Hammad narrated to us, from Qatadah and al-Qasim, both from Anas, that the Prophet, peace and blessings of Allah be upon him, said: "If the People of the Book greet you, then say: 'Alaykum (upon you)." And the other said: "Wa 'alaykum (and upon you)."