وَعَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: إِذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الكتابِ فَقولُوا: وَعَلَيْكُم
Anas (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When the People of the Book greet you with Salam, then you should reply: 'Wa Alaikum.'" Agreed upon.