Musnad Abu Ya'la al-Mawsili 115:339

Abu Musa narrated to us, Muhammad bin Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Qatadah narrating from Anas, that the companions of the Prophet (may Allah bless him and grant him peace) said to the Prophet (may Allah bless him and grant him peace): "Indeed, the People of the Scripture greet us, so how should we respond to them?" He said: "Say: 'Wa 'alaykum' (And upon you)."