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

((إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ)).

(When the people of the Book greet you, reply by saying: (And upon you.))