In the Sabha Parva of the Mahabharata, Draupadi’s question goes unanswered by the assembly. Vidura, waving his hands, silences the hall. “Ye are not answering her,” he says, in the Ganguli translation.
Then he states the rule. A person who knows the rules of morality and attends an assembly, but “doth not answer a query that is put, incurreth half the demerit that attacheth to a lie.”