USGA APPENDIX l; PART C; SECTION 1c

“Limitation on Balls Used During Round: (Note to Rule 5-1)"

(i) “One Ball” Condition

During a stipulated round, the balls a player plays must be of the same brand and model as detailed by a single entry on the current List of Conforming Golf Balls.

Note: If a ball of a different brand and/or model is dropped or placed it may be lifted, without penalty, and the player must then proceed by dropping or placing a proper ball (Rule 20-6). 

PENALTY FOR BREACH OF CONDITION: 

Match play – At the conclusion of the hole at which the breach is discovered, the state of the match is adjusted by deducting one hole for each hole at which a breach occurred; maximum deduction per round – Two holes.

Stroke play - Two strokes for each hole at which any breach occurred; maximum penalty per round- four strokes (two strokes at each of the first two holes at which any breach occurred).

(ii) Procedure When Breach Discovered 

When a player discovers that he has played a ball in breach of this condition, he must abandon that ball before playing from the next teeing ground and complete the round with a proper ball; otherwise, the player is disqualified. If discovery is made during play of a hole and the player elects to substitute a proper ball before completing that hole, the player must place a proper ball on the spot where the ball played in breach of the condition lay.”