‘Salām,’ or ‘Salām alaykum’ is a greeting which means ‘peace be with you’. In line with the overall ethos of the Quran and teachings of the prophetic sunna, not only is it permissible, rather it is encouraged to greet a non-Muslim in any way that falls under the overall etiquettes of greetings which includes 'Salām,’ or ‘Salām alaykum’. (Quran 4:86)