Ramban

THESE ARE THE GENERATIONS OF THE HEAVEN AND THE EARTH WHEN THEY WERE CREATED. Scripture now relates the account of the heaven and the earth as regards rain and growth after they had been created and put in proper order, that the heavens shall give their dew and rain, and the ground shall give her increase, these making possible the existence of all living beings. And in the word b’hibaram (when they were created) — [which could be read as if it were two words: b’hei baram] — Scripture alludes to what the Rabbis have said: “He created them with the letter hei” [which is the last of the four letters of the Tetragrammaton]. It is for this reason that Scripture until this point mentioned only the word Elokim. This is explained in the verse: For all these things hath My hand made;) of the Tetragrammaton is in the Cabala considered the yad hashem (the hand of G-d). See my Hebrew commentary, p. 32. and so did Job say, Who knoweth not among all these, that the hand of the Eternal hath wrought this? This being so, the expression, in the day that the Eternal G-d made, refers covertly to the word bereshith (in the beginning).