In his On Christian Liberty, Luther writes:
We should think of the works of a Christian who is justified and saved by faith because of the pure and free mercy of God, just as we would think of the works which Adam and Eve did in Paradise, and all their children would have done if they had not sinned. We read in Gen. 2[:15] that “The Lord God took the man and put him in the garden of Eden to till it and keep it.” Now Adam was created righteous and upright and without sin by God so that he had no need of being justified and made upright through his tilling and keeping the garden; but, that he might not be idle, the Lord gave him a task to do, to cultivate and protect the garden. This task would truly have been the freest of works, done only to please God and not to obtain righteousness, which Adam already had in full measure and which would have been the birthright of us all.
~pg. 38 (Fortress Press Facets edition)