Reference : Hisn al-Muslim 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 As-Salaamu `alaykum, then say (to him):

And upon you .

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/42, Muslim 4/1705.

Chapter: Returning a greeting to a disbeliever