[Rich Ives]

OBR 4.03:

When the ball is put in play at the start of, or during a game all fielders other than the catcher shall be on fair territory.


But there's no penalty specified.


The only penalty is for 4.03(a) Giving an IBB with the catcher out of the box - repeated in 8.05(l).


MLBUM (emphasis added)

2.16 FIRST BASEMAN PLAYING IN FOUL TERRITORY
Official Baseball Rule 4.03 provides that when the ball is put in play at the start of or during a game, all fielders other than the catcher shall be on fair territory. In particular, when holding a runner on first base, the first baseman shall position himself with both feet in fair territory. There is no penalty specified for violation other than the first baseman shall be instructed to keep both feet in fair territory if brought to the attention of the umpire, or-if blatant or recurring violationupon
immediate direction of the umpire. If a player, after so directed by the umpire, blatantly refuses to comply, the player is subject to ejection.



[Edited by Rich Ives on Oct 25th, 2005 at 03:28 PM]