The rule itself
At the moment a hand begins, whatever chips are in front of you are your entire stake for that hand. You can lose all of them and no more. You can win from each opponent no more than the amount you yourself have in play. Nothing outside the felt — cash in a pocket, a phone, an IOU, another player's stack — counts for anything until the hand is over. That is the whole rule, and every consequence below falls out of it without needing a separate rule of its own.
