[The Sabbath] is a creation ordinance. And let me say right away that, if I didn’t believe that it was a creation ordinance, then I’d have a hard time believing that the Sabbath was still an ordinance that had validity for us today. I think because it was a creation ordinance, that it has validity as long as we human beings experience time as a succession of days — until the real Sabbath comes, and it hasn’t — it doesn’t come until we enter into God’s Sabbath rest with him at last. But until that time, there is a place within the experience of God’s people for this wonderful, prophetic representation and promise of where it’s all heading. So because it’s a creation ordinance, I say it belongs to the whole process of the created order until we come to the consummation.
[Update] Wayne wins! The answer is OHS MGK, and you can hear for yourself (about 2 minutes in). Right after that quote, he continues, “Now, however, there are other features about it, that have to be noted.” Particularly, the feature that the Sabbath is an eschatological promise only for God’s people, thus unbelievers can in no wise observe the Sabbath. For more info, check out Lecture 5 here.