Musnad Abu Ya'la al-Mawsili 115:374

Ahmad narrated to us, Bahz narrated to us, Shu'bah narrated to us, Qatadah informed us, from Anas, who said: They said, "O Messenger of Allah, when the People of the Scripture greet us, how should we respond to them?" He said, "Say: 'Alaykum (Upon you).'"