كتاب الأدب
Read the English translation first, with the original Arabic directly underneath.
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)."
إِذَا سَلَّمَ عَلَيْكُمْ أَحَدٌ مِنْ أَهْلِ الْكِتَابِ فَقُولُوا وَعَلَيْكُمْ