33.2.1 A catch will be fair only if, in every case, either the ball, at any time, or any fielder in contact with the ball, is not grounded beyond the boundary before the catch is completed.
33.2.2 Furthermore, a catch will be fair if:
33.2.2.4 a fielder catches the ball after it has crossed the boundary in the air, provided that the conditions in 33.2.1 are met.
19.5.2 A fielder who is not in contact with the ground is considered to be grounded beyond the boundary if his/her final contact with the ground, before his/her first contact with the ball after it has been delivered by the bowler, was not entirely within the boundary.