I found it.
Ronald Knoxs A Detective Story Decalogue (1928)
is as follows:
One
The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.
Two
All supernatural or preternatural agencies are ruled out as a matter of course.
Three
Not more than one secret or passage is allowed.
Four
No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
Five
No Chinaman* must figure in the story.
*or other mysterious alien
Six
No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
Seven
The detective must not commit the crime.
Eight
The detective must not light on any clues which are not instantly produced for the inspection of the reader.
Nine
The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
Ten
Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.