Historical Rules · 5 October 2018
Rule 701.15a
Official source text from the 5 October 2018 Rules release
To reveal a card, show that card to all players for a brief time. If an effect causes a card to be revealed, it remains revealed for as long as necessary to complete the parts of the effect that card is relevant to. If the cost to cast a spell or activate an ability includes revealing a card, the card remains revealed from the time the spell or ability is announced until the time it leaves the stack.