From the USGA Rules of Golf:
The ball must be played as it lies, except as otherwise provided in the Rules.
(Ball at rest moved — see Rule 18.)
13-2. Improving Lie, Area of Intended Stance or Swing, or Line of Play
A player must not improve or allow to be improved:
• the position or lie of his ball,
• the area of his intended stance or swing,
• his line of play or a reasonable extension of that line beyond the hole, or
• the area in which he is to drop or place a ball,
by any of the following actions:
• pressing a club on the ground,
• moving, bending or breaking anything growing or fixed (including immovable obstructions and objects defining out of bounds),
• creating or eliminating irregularities of surface,
• removing or pressing down sand, loose soil, replaced divots or other cut turf placed in position, or
• removing dew, frost or water.