When you are near, your allies refuse to give up.

Encounter
Immediate Interrupt      Close burst 5

Trigger: An ally within 5 squares of you is reduced to 0 hit points or fewer by an attack

Target: The triggering ally in the burst

Effect: Until the end of the target's next turn, the dying condition doesn't cause the target to fall unconscious.

Published in Player's Handbook 3, page(s) 168.