Genesis 2:2-3

"And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His work which He had done."

"Then God blessed the seventh day and sanctified it, because in it He rested from all His work which God had created and made."

(God doesn't need rest. He rested on the seventh day for our sakes: that we might remember what He did, and follow His example. That's also why He sanctified it.)