How to testfor a player holding something and give him an effect

So i'm making a map that gives you a slime ball says REGEN and a sword. I would like to give the player holding the slime regen II for 1 second to heal if he's not fighting using /testfor. But how do I testfor and give to the exact player that holds the slime, as the regen may be given to another player who's not holding the slime.