Ex 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.

Ex 31:16,17  Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant. 

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.

Eze 20:12  Moreover also I gave them my sabbaths, to be a sign between me and them, that they might know that I am the LORD that sanctify them.

Eze 20:20  And hallow my sabbaths; and they shall be a sign between me and you, that ye may know that I am the LORD your God.