Oracle Text

Enchant creature

Enchanted creature can't attack unless its controller pays 3.

Card Rulings

10/4/2004 This ability is paid for during the declare attackers step of the combat phase.
10/4/2004 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.
10/4/2004 Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.