Sunan Ibn Majah, Volume 5, Book 33, Hadith 3697 (DarusSalam) ⓘ
It was narrated that Anas bin Malik said "the Messenger of Allah said": "When any of the People of the Book greets you with Salam (peace), then say, Wa 'alaikum(and also upon you)."
حَدَّثَنَا أَبُو بَكْرٍ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، وَمُحَمَّدُ بْنُ بِشْرٍ، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " إِذَا سَلَّمَ عَلَيْكُمْ أَحَدٌ مِنْ أَهْلِ الْكِتَابِ فَقُولُوا وَعَلَيْكُمْ " .
also referenced under: Hadith 3697 (Reference) & Book 33, Hadith 41 (In-Book)