Section 7: Violations & Penalties

Under Rule 7.1, a served ball is a fault if: The server infringes any part of Rule 6 ("The Service") — including failing to keep at least one foot behind the service line until the ball is struck, touching the service line or central line with a foot, or letting the ball cross the service line (o...

Source: Rule 7, Rule 7.1, Rule 6, Rule 6.1, Rule 13.1

Send this to your friend — argument over.