Laws of Cricket 1947 Code (Fourth Edition) - Law 38
The Striker is out "Hit wicket" - If in playing at the ball he hit down his wicket with his bat or any part of his person. (Experimental additional words to the Law)
1. At any time when playing at the ball.
2. In setting out for his first run.
Notes on Law 38
1. "When playing at the ball" shall include a second strike by the batsman to keep the ball out of his wicket.
2. Any part of the batsman's dress will be considered part of his person.
3. A batsman is "Not Out" if he breaks his wicket in avoiding being run out or stumped.
Reproduction of the Laws of Cricket is by kind permission of Marylebone Cricket Club