The original text of this sentence is:

When you rule out the impossible, the rest, no matter how impossible, must be true.

It comes from:

Symbol of four (1980)

"You won't apply my rules," he said, shaking his head. "How many times have I told you, when you rule out the impossible, the rest, no matter how incredible, must be true?

We know that he didn't come in through the door, window or chimney.

We also know that he can't be hidden in the room, right there.

There is no possibility of hiding.

So, when did he come? "

-the symbol of four, ch. 6 ( 1890)

-Sherlock Holmes in "Four Constellations" (double day P.11)