SpaceX recently launched the Falcon Heavy into space while carrying Elon Musk's car to be sent into orbit around Mars. (News article reference) During the launch, footage was shown of a space suit wearing mannequin in the front seat of the Tesla, while earth is visible in the background.
As you can see, around the dashboard of the car there are the words "DON'T PANIC!" and I am sure it is a reference to something sci-fi related but can't think of what.
My question is: What is this a reference to?