Kommentar zu Bereschit 5:18

ויולד את חנוך, “he begat Chanoch.” This was the seventh generation, a number that was predestined for holiness and sanctity. David is also mentioned as having been the seventh son of his father Yishai, although that had been obvious from the way Samuel had been looking for the man to replace King Shaul. (Chronicles I 2,15) The number “seven” is special in the seven “heavens,” the seven deserts the Jewish people traversed, i.e. the desert of Sinai, in the seven seas, i.e. Lake Tiberias. [The subject has been expanded upon in Pirkey de Rabbi Eliezer, chapter 18. Ed.] The seventh universe (which according to our tradition, will be the last and perfect universe), as well as the seventh day of Creation, are all examples of the significance of the number “seven.”