The Messenger of Allah ﷺ said, "When the People of the Book greet you, respond by saying, 'And upon you.'"
حدثنا هشيم أنبأنا عبيد الله بن أبي بكر بن أنس
قال رسول الله ﷺ إذا سلم عليكم أهل الكتاب فقولوا وعليكم
Its isnad is sahih. Bukhari related it, 8/71, in the chapter on seeking permission, "How to respond to the People of the Covenant," and Tirmidhi, 5/407, number 3301, in the tafsir of Surah Mujadilah, saying, hasan sahih, and Ibn Majah, 2/1219, number 3697.I once heard some people who are counted among the scholars declare this hadith daif, on the grounds that Islam does not distinguish between citizens, in order to deny the accusation that Muslims make the People of the Covenant second-class citizens. We say, yes, they are second-class, but have we ever slaughtered any of them? As for those who do not announce the status of Muslims in their own countries, have the massacres ended, even to this day? It is the height of ignorance for us to disown our religion in order to please the disbelievers, and they will never be pleased with us, as stated by the text of the wise Quran.