"Prose is architecture, not interior decoration," said Hemingway, "and the baroque is over." His own style was without ornament.
He also decided, as "a good and severe discipline" to write one story about each thing he knew. This may be the original source of the dictum to "Write what you know." Yet to slavishly follow this much misunderstood "advice" would deprive the world of a great deal of imaginative work.
In his 1960 introduction to A Moveable Feast, Hemingway hints at the blurry line between fact and fiction:
"If the reader prefers, this book may be regarded as fiction. But there is always the chance that such a book may throw some light on what has been written as fact."
Hemingway was a slow writer who said this: "I did not know how I would ever write anything as long as a novel. It often took me a full morning of work to write a paragraph."
Hemingway's advice for writing can be found here.