Charles Ellicott Commentary Exodus 31:17

"It is a sign between me and the children of Israel for ever: for in six days Jehovah made heaven and earth, and on the seventh day he rested, and was refreshed." — Exodus 31:17 (ASV)

For in six days the Lord made heaven and earth. Whatever other grounds there were for Sabbath observance, this idea always lay at its root. Man was through it to be made like his Maker—to have from time to time a rest from his labours, as God had had (Genesis 2:2–3)—and thereby to realise the blessedness of that final rest which he may be sure remaineth for the people of God.