How to reply to a disbeliever if he says Salam to you

227. If one of the People of the Scripture (i.e. Christians and Jews) greets you saying As-Salaamu 'alaykum, then say (to him):

Wa 'alaykum

And upon you .

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