Rule 19: Ball in motion deflected or stopped.

Definition.

An "outside agency" is any agency not part of the match or, in stroke play, not part of the competitor's side, and includes a referee, a marker, an observer and a forecaddie. Neither wind nor water is an outside agency.

 

19-1 By outside agency.

If a ball in motion is accidentally deflected or stopped by any outside agency, it is a rub of the green, there is no penalty and the ball must be played as it lies, except:
(a) If a ball in motion after a stroke other than on the putting green comes to rest in or on any moving or animate outside agency, the player must, through the green or in a hazard, drop the ball, or on the putting green place the ball, as near as possible to the spot where the outside agency was when the ball came to rest in or on it;
(b) If a ball in motion after a stroke on the putting green is deflected or stopped by, or comes to rest in or on, any moving or animate outside agency except a worm or an insect, the stroke is canceled. The ball must be replaced and the stroke replayed. If the ball is not immediately recoverable, another ball may be substituted.