Fortress of the Muslim (Hisn al-Muslim)
حصن المسلم
(109) Chapter: Returning a greeting to a disbeliever
(109) كيف يرد السلام على الكافر إذا سلم
Hisn al-Muslim 227 
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 As-Salaamu `alaykum, then say (to him): And upon you . Reference: Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/42, Muslim 4/1705.
إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ
Reference : Hisn al-Muslim 227