The detailed analysis of a conflict leading to a disciplinary procedure serves as a main thread for delving into the manner in which actors in a French prison mobilize and use the law. More broadly, this study examines the growing impact of rights resulting from recent reforms. The case examines how the disciplinary procedure and, more significantly, the rule of law have been put to the test through a dispute concerning a real or supposed prayer performed by a Muslim detainee in a place where prayer is forbidden. Examining the reasons why a guard decided to use a bureaucratic sanction and how the detainee succeeded in forcing his adversary into taking his rights seriously, this case highlights the contingency of legal recourse and how law is performed in prison. Judicial control offers new opportunities for detainees to resist (here, successfully), but it tends to disrupt the guards’ professional culture. In this instance, laws and rights appear to be instruments and weapons rather than rules to pacify the relationship in prison.