In this office, silence is the 11th commandment.
11th commandment
idiomA well-known convention (often the most well-known of a certain field) which supposedly can not or should not be broken.
Origin
disputed originIn reference to the biblical Ten Commandments, carrying the implication that the convention is of very high importance (similar to the Ten Commandments).
He ignored the 11th commandment of the industry by speaking openly about the company's secret financial problems.
Although the team claimed to value innovation, they treated the founder's original design philosophy as an 11th commandment that no one dared to challenge.