Section 5: Rules of Play

The serve must be made underhand. The server must bounce the ball on the ground and strike it at or below waist height.

Source: Rule 6, Rule 6.1, Rule 6.2, Rule 6.3, Rule 6.4, Rule 6.5, Rule 9.1, Rule 7.1, Rule 13.1

Send this to your friend — argument over.