Historical Rules · 8 April 2016
Rule 401.5
Official source text from the 8 April 2016 Rules release
Some effects tell a player to play with the top card of his or her library revealed, or say that a player may look at the top card of his or her library. If the top card of the player’s library changes while a spell is being cast, the new top card won’t be revealed and can’t be looked at until the spell becomes cast (see rule 601.2i). The same is true with relation to an ability being activated.