Historical Rules · 1 June 2014
Rule 701.13a
Official source text from the 1 June 2014 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 it the time it leaves the stack.