Resting from work on Shabbat is our way of proclaiming and attesting that God created the world, and that He rested on Shabbat.

It is written: “Remember the Sabbath day, to keep it holy” (Exodus 20:8); corresponding to it, “You shall not bear false witness” (Exodus 20:13) is written. The Torah is stating that whoever profanes Shabbat testifies before the One who spoke and the world came into being that He did not create His world in six days and did not rest on the seventh day. And whoever observes Shabbat testifies before the One who spoke and the world came into being that He did create His world in six days and rested on the seventh day, as it is stated: “You are My witnesses – the utterance of the Lord – and I am God” (Isaiah 43:12). (Mekhilta deRabbi Yishmael, Yitro, Mesekhta deBaĥodesh 8)