14 CFR § 121.597

No person may start a flight under a flight following system without specific authority from the person authorized by the operator to exercise operational control over the flight.

No person may start a flight unless the pilot in command or the person authorized by the operator to exercise operational control over the flight has executed a flight release setting forth the conditions under which the flights will be conducted. The pilot in command may sign the flight release only when he and the person authorized by the operator to exercise operational control believe that the flight can be made with safety.

No person may continue a flight from an intermediate airport without a new flight release if the aircraft has been on the ground more than six hours.