Matthew Poole and continuators on Exodus 31:15

15 Six days may work be done, but in the seventh is the sabbath of rest : holy to the LORD: whosoever doeth any work in the sabbath day, he shall surely be put to death.

Heb. the sabbath of sabbaths, or of sabbaths, i. e. the great and chief sabbath, as the song of songs is the most excellent song, the holy of holies is the most holy, &c. The Iews had many sabbaths or days of rest, but this is here preferred before them all by this emphatical repetition of the same word, and by this argument the foregoing duty is pressed upon them.

Marginal references: Heb. shall be done.; Heb. holiness.