Does a fielder who catches a foul ball in the air have to land in the field of play? I.E. when player chases a foul flyball, then catches ball leaping and landing in seats/dugout... Out or Foul ball ?
The MLB rulebook does mention that the fielder must stay in the field of play for the ball to remain in play. The ball is dead if this doesn't happen, but that does not invalidate any catch.
From the first comment on 5.09(a)(1)
Ball is in play, unless the fielder, after making a legal catch, steps or falls into a dugout or other out-of-play area, in which case the ball is dead. Status of runners shall be as described in Rule 5.06(b)(3)(C) Comment.
A further comment on the same rule confirms that falling into the dugout does not invalidate a catch. It should therefore be presumed that falling out of the field of play would not invalidate it either.
If a fielder, attempting a catch at the edge of the dugout, is “held up” and kept from an apparent fall by a player or players of either team and the catch is made, it shall be allowed.