What is the ruling on returning the greeting of "peace" by saying: "And upon you be peace, concerning what you said"?

If a Muslim initiates a greeting, their greeting must be returned in kind or with something better. The minimum response is "Wa alaikum assalam," and adding "ala ma qult" (on what you said) to it is considered an innovation (bid'ah) because it contradicts the guidance of the Prophet, peace and blessings be upon him.

However, if the one who initiates the greeting is a non-Muslim, such as a Jew or a Christian, it is permissible to respond with "Wa alaikum" (And upon you) or "Alaika ma qult" (Upon you what you said), based on the saying of the Prophet, peace and blessings be upon him: "If one of the People of the Book greets you, then say: 'Upon you what you said.'"