… is from page 314 of UCLA economist William Allen’s 1989 collection of the transcripts of his insightful and still-germane radio addresses, The Midnight Economist; specifically, it’s from Allen’s April 1988 radio address titled “Untruths About the Balance of Payments” (original emphasis):
It is not true that the balance of payments is unbalanced. The [U.S.] balance of payments is a double-entry accounting record of transactions of United States residents with the rest of the world. Since each transaction is balanced, then over any period the total of transactions necessarily balances.