How to reply to a disbeliever if he greets you

The Messenger of Allah ﷺ said: "When the People of the Book greet you [with the words "as-samu ‘alaykum"¹], say in reply:"

"And upon you."

¹ "As-samu ‘alaykum" means "may death be upon you." Twisting the usual greeting "as-salamu ‘alaykum," the Jews of Medina used the similar-sounding word "sam" (death), which is what this hadith refers to. The reply "wa ‘alaykum" (without "salam") simply returns the wish upon the one who spoke it.

[al-Bukhari 6258; Muslim 2163]