The R&A Rules of Golf state the following:

"Rule 14 - Striking the Ball:

In making a stroke, a player must not:
a. accept physical assistance or protection from the elements; or
b. allow his caddie, his partner or his partner's caddie to position himself on or close to an extension of the line of play or the line of putt behind the ball.
PENALTY FOR BREACH OF RULE 14-1 or 14-2:
Match play - Loss of hole; Stroke play - Two strokes"

Hope this helps.