The quotation is from the beginning of "Nam Quốc Sơn Hà," a famous 10th century poem considered Vietnam's first declaration of independence (from China). Written in classical Chinese, the Vietnamese version quoted here was translated by the great Trần Trọng Kim.
The Southern rivers and mountains, the Southern king lives
It's clearly stated in Heaven's book
How dare these invaders trespass
They will be smashed to pieces!