Isaiah 56:2, 6

Blessed is the man that doeth this, and the son of man that holdeth it fast; that keepeth the sabbath from profaning it, and keepeth his hand from doing any evil.

Also the foreigners that join themselves to Jehovah, to minister unto him, and to love the name of Jehovah, to be his servants, every one that keepeth the sabbath from profaning it, and holdeth fast my covenant