the salām, then al-Bukhārī (6258) and Muslim (2163) relate on the authority of Anas that the Prophet (peace and blessings be upon him) said: “When the People of the Book greet you with salām, say: Wa-ʿalaykum (and upon you).”

Including the particle wa (“and”) in the reply is the position held by the majority of scholars, although some have opted to omit it. The explicit wording of the hadith is the safest course to follow, and Allah knows best.