Merric is right. That rule doesn't exist in quite so erudite a form as you claim, MitFH. It can be derived by assuming a few things, but that rule was never codified until 3e.
Erm, it's in the original D&D books (Men & Magic) and in Basic D&D (Moldvay) - probably in Holmes as well, but I don't know that version so well.
It just isn't in AD&D 1E.
Cheers!