Something like Mana Flare? Thats not really passive though.... make a custom aura with a custom buff then:
Event: A unit begins finishes casting a spell,
Conditions: (Triggering Unit) has a Buff equal to (Buff emitted from your custom Aura)
Actions: Whatever you want
I checked teh trigger editor and those are real conditions you can use so the trigger "should" work if you don't mess up on the Actions. GL and tell me if this works. I would show an actual trigger script but I don't know what button to press to get a picture of triggers in, sry.