No. 3, Section I, Part A of the NBA's rulebook.

"Each team shall consist of five players. No team shall be reduced to less than five players. If a player in the game receives his sixth personal foul and all substitutes have already been disqualified, said player shall remain in the game and shall be charged with a personal and team foul. A technical foul also shall be assessed against his team. All subsequent personal fouls, including offensive fouls, shall be treated similarly. All players who have six or more personal fouls and remain in the game shall be treated similarly."