You point your bow at the villain who just wounded your friend and loose a vengeful arrow.

Encounter   ✦     Martial, Weapon
Immediate Reaction      Ranged weapon

Trigger: A creature within range attacks your ally

Target: The attacking creature

Attack: Dexterity vs. AC, and you gain a power bonus to this attack equal to your Wisdom modifier.

Hit: 2[W] + Dexterity modifier damage.

Published in Player's Handbook, page(s) 110.