The taking, receiving, reserving, or charging a rate of interest greater than is allowed by section 85 of this title, when knowingly done, shall be deemed a forfeiture of the entire interest which the note, bill, or other evidence of debt carries with it, or which has been agreed to be paid thereon. In case the greater rate of interest has been paid, the person by whom it has been paid, or his legal representatives, may recover back, in an action in the nature of an action of debt, twice the amount of the interest thus paid from the association taking or receiving the same: Provided, That such action is commenced within two years from the time the usurious transaction occurred.
12 U.S. Code § 86 - Usurious interest; penalty for taking; limitations
(R.S. § 5198.)