Romans 4:6

David says the same thing when he speaks of the blessedness of the one to whom God credits righteousness apart from works:

(v6, NIV)

This train of thought may be quite different from the Jews at the time. However, Paul uses an example from David to reinforce the legitimacy of what he's talking about. David in the Old Testament does in fact talk about a God who credits righteousness in a way different from works. Though it may seem to many that the Old Testament teaches works and observance of the law as a means to credit righteousness, David seems to say differently in the passage Paul is about to quote.