The 174th mitzvah is that we are commanded to obey the Beis Din HaGadol and act in accordance with all their instructions regarding what is prohibited and what is permitted. There is no difference whether it is something they received by Oral Tradition; derived using one of the principles of Torah extrapolation; decreed in order to correct some laxity or in response to some other situation where they found it appropriate and that it would strengthen Torah observance. We are required to obey all such directives and to act in accordance with their words, not to transgress them.
The source of this commandment is G‑d's statement1 (exalted be He), "You must keep the Torah as they interpret it for you [and follow the laws that they legislate for you]."
The Sifri says, "The verse, 'Follow the laws that they legislate for you' constitutes a positive commandment."
The details of this mitzvah are explained in the end of tractate Sanhedrin.2
