What law protects religious dress codes in the work place?

I’m writing a paper for an Employment Law class. I need to know the specific act or law or something that which gives employees the right to wear certain clothing if it’s a religious requirement.

My prompt is to write about legalities of dress codes for a company that is writing a dress code. I want to cover the religious aspect.

Any help will be appreciated! But anything I state I need to back up with a law.