What does Paul mean when he says "it is written"?
When Paul says "it is written," he means he is pointing back to the Old Testament Scriptures. The phrase introduces an authoritative quotation or reference from the Hebrew Bible, as when he cites the account of Abraham's two sons in Genesis to make his doctrinal point.
Les explains that whenever Paul uses the term "it is written," he is going to take his readers back to the Old Testament. In this passage, Paul is addressing those who desire to be under the Law and asks if they have not heard the Law. Then he writes, "For it is written," and proceeds to the account of Abraham having two sons, one by a bondmaid and the other by a free woman. Les points out this takes us back to Genesis, where Abram had waited 36 years for the promise of a son, and Sarai had no sign of being able to have a child. So Paul's phrase signals an appeal to the Old Testament Scriptures to ground his argument.
Tell me, ye that desire to be under the law, do ye not hear the law?