Historical Rules · 26 August 2016
Rule 701.7c
Official source text from the 26 August 2016 Rules release
If a card is discarded, but an effect causes it to be put into a hidden zone instead of into its owner’s graveyard without being revealed, all values of that card’s characteristics are considered to be undefined. If a card is discarded this way to pay a cost that specifies a characteristic about the discarded card, that cost payment is illegal; the game returns to the moment before the cost was paid (see rule 720, “Handling Illegal Actions”).