ALWAYS "prescription." NEVER "receipt."
Sometimes, informally, a prescription is called a "script." And sometimes it's referred to by the abbreviation "Rx."
In U.S. English there is an OUTDATED meaning of the word "receipt" to mean a COOKING recipe. I see that ahdictionary.org gives it as the fourth meaning for "receipt." It also says that "receipt" is from a medieval Latin word "receta" which means BOTH a medical prescription AND money received. So the history of the word "receipt" is tangled.