Al-Kahf 18:20

Innahum in yathharoo AAalaykumyarjumookum aw yuAAeedookum fee millatihim walan tuflihooithan abada

Indeed, if they come to know of you, they will stone you or return you to their religion. And never would you succeed, then - ever."

If they (the habitants of the city) will know about you, they will force you to revert to their faith, and in that case, you will never find success.”