Sefer HaBahir

[Rabbi Berachiah continued,] "Why is it written, 'The light of the moon will be like the light of the sun, and the light of the sun will be sevenfold, like the light of the seven days [שבעת הימים]' (Isaiah 30:26). 'The days [הימים]'? It should have said 'days [ימים]'! But these are the days about whom it is written, 'Because YHVH made six days' (Exodus 31:17)." His friend said to him, "This, I have received from what is written, 'Because YHVH made six days', like that which you said, 'The Blessed Holy One made six beautiful vessels.' What are they?"[He continued,] 'The Heavens and the Earth' (Exodus 31:17, Genesis 1:1)."[His friend objected,] "But they are seven!"[He continued,] "And therefore, it is written, 'On the seventh day, [God] stopped and rested [וינפש]' (Exodus 31:17). What is 'rested [וינפש]'? It teaches that the day of Shabbat establishes all souls [הנפשות], as is said, 'and [He] ensouled [וינפש].'"