Exodus 31:13,17

31:13  Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it [is] a sign between me and you throughout your generations; that [ye] may know that I [am] the LORD that doth sanctify you.

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.