Romans 3:20
This is because no human being can be considered right with God by [obeying] the requirements of a law. [Note: It is difficult to determine the exact sense in which "law" is used in these verses. It may be law generally, or the law of Moses specifically. See Bruce, pages 52-58]. For the knowledge of what sin is comes from the law of Moses.
Romans 7:7-11
What shall we say then? Is the law of Moses sinful? Certainly not! For I would not have known what sin was if it had not been for the law. For [example]; I would not have known [what it meant] to covet [i.e., have a strong desire for what belongs to someone else] except that the law said [Ex. 20:17], "You must not covet."
Galatians 2:19
For it was by means of [this system of] law-keeping that I [realized I] was dead. This was so I could find life through a relationship with God.
Galatians 3:10-13
For all who seek to be right with God by trying to meet all the requirements of the law of Moses are under a curse [from God], because it is written [Deut. 27:26], "Every person is cursed [i.e., cut off from God] who does not continue to obey all of the requirements written in the law of Moses."