Musnad Ahmad 32:1376

Hashim narrated to us, Shu'bah narrated to us, from Qatadah, who said: Anas ibn Malik said: The companions of the Messenger of Allah, peace and blessings of Allah be upon him, said: "O Messenger of Allah, the People of the Scripture greet us, so how should we respond to them?" He said: "Say: 'Wa 'alaykum' (And upon you)."