Q: What are the reasons annexed to the fourth commandment?
A: The reasons annexed to the fourth commandment are, God's allowing us six days of the week for our own employments, his challenging a special propriety in the seventh, his own example, and his blessing the Sabbath-day.

Exodus 31:15-16. Six days may work be done; but in the seventh is the sabbath of rest, holy to the LORD: whosoever doeth any work in the sabbath day, he shall surely be put to death. Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant.
Leviticus 23:3. Six days shall work be done: but the seventh day is the sabbath of rest, an holy convocation; ye shall do no work therein: it is the sabbath of the LORD in all your dwellings.
Exodus 31:17. It is a sign between me and the children of Israel for ever: for in six days the LORD made heaven and earth, and on the seventh day he rested, and was refreshed.
Genesis 2:3. And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.

Westminster Project - The Shorter Catechism explained and proven by Scripture by Thomas Vincent