Sayyiduna Anas (may Allah be pleased with him) narrates: We were forbidden (the narrator is unsure; perhaps the words are:) we were commanded that we should not say to the People of the Book (when replying to their salam) anything more than "Wa alaykum (And upon you)."
Imam Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12792