Musnad Ahmad 32:1144

Yazid narrated to us, Shu'bah informed us, and Muhammad ibn Ja'far narrated to us, Shu'bah said: I heard Qatadah narrating from Anas ibn Malik, that the companions of the Messenger of Allah, may Allah bless him and grant him peace, said to the Messenger of Allah, may Allah bless him and grant him peace: "The People of the Book greet us, so how should we respond to them?" He said: "Say: 'Wa 'alaykum' (And upon you)."