Failure to replace the ball that was moved

It could mean the shot in question was a tee shot, or an approach played into a
green, or any other scenario other than when both balls were on the green prior to
making the stroke. Failure to replace the ball that was moved to its original spot;
or moving the ball that did the striking (rather than playing it as it lies) results
in loss of hole in match play or a 2-stroke penalty in stroke play.
There is no penalty to either player as long as this procedure is followed: The
person whose shot struck the ball at rest plays his ball as it lies; the person
whose ball was moved returns the ball to its original position.For example, your
partner hits a tee shot, then you hit yours, and your ball hits your partner's ball
in thefairway. Or your partner is on the green, you hit an approach shot, and your
ball hits your partner's on the green.
This is covered in Rule 18-5, Ball at Rest Moved by Another Ball.