"And on the seventh day God finished His work which He had made; and He rested on the seventh day from all His work which He had made." Genesis 2:2
Some understand that this verse teaches that Shabbat was instituted at creation, and is therefore a universal moral requirement (for all people, and all time). It follows from this assumption that Shabbat would have been observed by Adam, Enoch and Noah.
Please comment on the accuracy of this, or point me to a Jewish scholarly work that would be able to set this issue straight.