(13) باب رَدِّ السَّلاَمِ عَلَى أَهْلِ الذِّمَّةِ
It was narrated that Anas bin Malik said "the Messenger of Allah(ﷺ) said":
"When any of the People of the Book greets you with Salam (peace), then say, Wa 'alaikum(and also upon you)."
Reference: Sunan Ibn Majah 3697
In-book reference: Book 33, Hadith 41
English translation: Vol. 5, Book 33, Hadith 3697