The Official Site of Major League Baseball: Official info: Official Rules

Rule 7.04(c) Comment: If a fielder, after having made a legal catch, should fall into a stand or among spectators or into the dugout or any other out-of-play area while in possession of the ball after making a legal catch, or fall while in the dugout after making a legal catch, the ball is dead and each runner shall advance one base, without liability to be put out, from his last legally touched base at the time the fielder fell into, or in, such out-of-play area.