Rule 19: Ball in motion deflected or stopped.

19-2 b Stroke play.

If a competitor's ball is accidentally deflected or stopped by himself, his partner or either of their caddies or equipment, the competitor incurs a penalty of two strokes. The ball must be played as it lies, except when it comes to rest in or on the competitor's, his partner's or either of their caddies' clothes or equipment, in which case the competitor must through the green or in a hazard drop the ball, or on the putting green place the ball, as near as possible to where the article was when the ball came to rest in or on it.

 

19-5 By another ball.

(a) At rest.
If a player's ball in motion after a stroke is deflected or stopped by a ball in play and at rest, the player must play his ball as it lies. In match play, there is no penalty. In stroke play, there is no penalty unless both balls lay on the putting green prior to the stroke, in which case the player incurs a penalty of two strokes.
(b) In motion.
If a player's ball in motion after a stroke is deflected or stopped by another ball in motion after a stroke, the player must play his ball as it lies. There is no penalty unless the player was in breach of rule 16- 1f, in which case he incurs the penalty for breach of that rule. Exception: If the player’s ball is in motion after a stroke on the putting green and the other ball in motion is an outside agency.