Discards touch your hole cards, where's the rule?

[Zetack]

I've heard it definitively stated on a number of occaisions that if somebody mucks their hand onto your hole cards and you aren't protecting your cards, your hand is dead too.

I see in Robert's rules that the thrown cards are dead (which seems superfluous), but I don't see any rule that the player who failed to protect his cards has a dead hand as well.

Is this a rule or a just a common misconception? Where's the support for it is a rule?

Edit: Crud, I see there's a thread on this "my cards get mucked, standard?" already. Mods, feel free to move this post to that thread or kill this thread. In the meantime, I don't see the answer there, so if anybody knows, feel free to let me know the answer.