Fervor
Enchantment
Creatures you control have haste. (They can attack and 
as soon as they come under your control.)
The intensity of their glare was soon surpassed by the intensity of their attack.
Illustrated by
Seventh Edition#181RareEnglishLegality
Rulings
Jul 1, 2012
If an attacking creature loses haste, perhaps because Fervor leaves the battlefield after attackers have been declared, it won’t be removed from combat.