International Institute of Cricket Umpiring & Scoring
Run out from reboundIn a local league match, the striker hit the ball and it struck the helmet of the short leg fielder. The ball then rebounded directly onto the striker's wicket but the striker was still in his ground. There was an appeal and the umpire called "Not out". In the bar after the match there was a discussion about the incident and the question was posed - "would the striker have been out if he had left his ground?"
No, he would not; because the ball did not come into contact with any member of the fielding side save for the contact with the helmet. The ball was not dead because it had struck the helmet and either batsmen can be Run out if a wicket is subsequently put down by a fielder correctly.
Had the helmeted fielder, or any other fielder, touched the ball in its passage from bat to wicket via helmet with any part of his person and if the striker had been out of his ground when the wicket was put down, then he would have been out Run out (Law 38.2(c)).
Read more about Law 38 (Run out) at the MCC website