D.C. Code § 15-110: Interest on judgment on contracts made elsewhere.

In an action on a contract for the payment of a higher rate of interest than is lawful in the District, made or to be performed in a State or territory of the United States where such a contract rate of interest is lawful, the judgment for the plaintiff shall include the contract interest to the date of the judgment and interest thereafter at the rate of 6 per cent per annum until paid.