Exodus 31:16-18

Young's Literal Translation

¹⁶ and the sons of Israel have observed the sabbath; to keep the sabbath to their generations [is] a covenant age-during,

¹⁷ between Me and the sons of Israel it [is] a sign -- to the age; for six days Jehovah made the heavens and the earth, and in the seventh day He hath ceased, and is refreshed.'

¹⁸ And He giveth unto Moses, when He finisheth speaking with him in mount Sinai, two tables of the testimony, tables of stone, written by the finger of God.