Law 9 - The Ball In and Out of Play
Source - FIFA.com
Ball out of play
The ball is out of play when:
- it has wholly crossed the goal line or touch line whether on the ground or in the air
- play has been stopped by the referee
Ball in play
The ball is in play at all other times, including when:
- it rebounds off a goalpost, crossbar or corner flagpost and remains in the field of play
- it rebounds off either the referee or an assistant referee when they are on the field of play