Brainwash

Enchantment — Aura

Enchant creature
Enchanted creature can't attack unless its controller pays .

2004-10-04:
This ability is paid for during the declare attackers step of the combat phase.
2004-10-04:
If there are multiple combat phases during the turn, the attack cost must be paid each time if you want to attack with the creature.
2004-10-04:
Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.