Player position in fair/foul territory

Question
I am trying to instruct my first and third baseman on how to hold a runner on base.  I was told by one umpire that the defensive player must have both feet in fair territory and another that they just had to have one foot in fair and the rest of their body can be in foul territory.  What is the rule and is their a penalty for an infraction?

Answer
Todd,

It depends on what ruleset your league is using.

With that said, there is no penalty in any ruleset. After being warned, the player will be ejected from the game. There's no other penalty.

Further, this rule is the same as the "base coaches must remain in the coach's box" rule. The ONLY time it's enforced, except in gross errors (standing behind the catcher, for example) is after another team complains.