Galatians 3:11-12
11 But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith.
12 And the law is not of faith: but, The man that doeth them shall live in them.
The Grace New Testament Commentary - Galatians 3:12
3:12 Yet the law is not of faith. Under the Law of Moses a person's life was contingent on obedience: "the man who does them shall live by them" (Lev 18:5). Faith in the Law did not extend one's physical life. Obedience did.
If the Law was to be the means of justification, it would have to be by faith. But since the law is not of faith, it clearly is not a means of justification before God. This argument not only excludes the Law of Moses; it also excludes anything that is not of faith, that is, anything that is of works.