Hisn al-Muslim · Book 1 · Hadith 227
إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ
If one of the People of the Scripture (i.e. Christians and Jews) greets you, saying As-Salāmu `alaykum, then say (to him): Wa `alaykum. If one of the People of the Scripture (i.e. Christians and Jews) greets you saying…