Historical Rules · 17 July 2015
Rule 508.1g
Official source text from the 17 July 2015 Rules release
If any of the chosen creatures require paying costs to attack, the active player determines the total cost to attack. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, ignore this change.