Laws of the Game
4. Offences and sanctions
If an offside offence occurs, the referee awards an indirect free kick where the offence occurred, including if it is in the player’s own half of the field of play.
A defending player who leaves the field of play without the referee’s permission shall be considered to be on the goal line or touchline for the purposes of offside until the next stoppage in play or until the defending team has played the ball towards the halfway line and it is outside its penalty area. If the player left the field of play deliberately, the player must be cautioned when the ball is next out of play.
An attacking player may step or stay off the field of play not to be involved in active play. If the player re-enters from the goal line and becomes involved in play before the next stoppage in play or the defending team has played the ball towards the halfway line and it is outside its penalty area, the player shall be considered to be positioned on the goal line for the purposes of offside. A player who deliberately leaves the field of play and re-enters without the referee’s permission and is not penalised for offside and gains an advantage must be cautioned.
If an attacking player remains stationary between the goalposts and inside the goal as the ball enters the goal, a goal must be awarded unless the player commits an offside offence or a Law 12 offence, in which case play is restarted with an indirect or direct free kick.