What Old Testament passage is Paul referring to in Galatians when he speaks of the curse of the Law?

Les teaches that when a person says he will keep the Law in order to merit favor with God, there is no way any mortal man can do it, so he puts himself under the curse. The passage Paul refers to is Deuteronomy 27, which says, “Cursed be he that confirmeth not all the words of this law to do them.” This means that if you do not keep every last jot and tittle of the Law, as no man can do and only God can do, you are under the curse of God. Les then flips back to Galatians 3, saying this is exactly what Paul is making reference to: “for as many are on the works of the Law unto the curse, for it is written, Cursed is every one that continueth not in all the things which are written in the book of the Law to do them.” It is a tremendous warning: don’t try to put yourself under the Law, since you are human and cannot keep it anyway. When you put yourself under a so-called legal system, you are actually putting yourself under the curse of God. Les calls this serious business and serious language, and says it is not his, it is the Book.