LMAO..they call it Arnie's Army...lol..story goes they use to love Arnold Palmer so much, that if missed the fairway, people would kick his ball back into play, and if he was missing the green they would jump in front of the ball to make it bounce onto the green.

Generally speaking rule 18 covers balls, specifically what you are talking about is

18-1 By Outside Agency;If a ball at rest is moved by an outside agency, there is no penalty and the ball must be replaced.
(Player’s ball at rest moved by another ball — see Rule 18-5.)

or
as Lowpost said:

Rub of the Green
A “rub of the green’’ occurs when a ball in motion is accidentally deflected or stopped by any outside agency (see Rule 19-1).

You can goto this URL to see all the rule 18 covering balls:
http://www.usga.org/playing/rules/bo...le18.html#18-1