Det kommer från The Hitchhiker's Guide to the Galaxy .
Det har även sin egen post på Wikipedia-sidan för H2G2:
In the series, Don't Panic is a phrase on the cover of The Hitchhiker's Guide to the Galaxy. The novel explains that this was partly because the device "looked insanely complicated" to operate, and partly to keep intergalactic travellers from panicking.
Från den första boken i serien:
It is said that despite its many glaring (and occasionally fatal) inaccuracies, the Hitchhiker's Guide to the Galaxy itself has outsold the Encyclopedia Galactica because it is slightly cheaper, and because it has the words 'DON'T PANIC' in large, friendly letters on the cover.