First Basemans position on pick off

Question
What is the rule for the position of the first baseman on a pick off?  I understand the first baseman must have both feet inside the field of play.  Please clarify my understanding and please list the specific rule.  Thank you.

Answer
Actually, only one foot inside the field of play is necessary. Section 4.03 of the Official Baseball Rules says that each player shall be in fair territory -- and having one foot in satisfies that requirement. However, I don't personally think the best way to position yourself is by straddling the base. I prefer being on the inside corner of the bag when holding runners on, which is a better position for fielding if the ball should be hit your way. I believe it's also easier to put down a tag on a pick-off from that position.