Law 28 (The wicket is down)
1. Wicket put down
(a) The wicket is put down if a bail is completely removed from the top of the stumps, or a stump is struck out of the ground by
(i) the ball.
(ii) the striker's bat, whether he is holding it or has let go of it.
(iii) the striker's person or by any part of his clothing or equipment becoming detached from his person.
(iv) a fielder, with his hand or arm, providing that the ball is held in the hand or hands so used, or in the hand of the arm so used.
The wicket is also put down if a fielder pulls a stump out of the ground in the same manner.
(b) The disturbance of a bail, whether temporary or not, shall not constitute its complete removal from the top of the stumps, but if a bail in falling lodges between two of the stumps this shall be regarded as complete removal.