זוהר ויקרא — דף יד ע”ב

Rabbi Shimon teaches Rabbi Elazar that one who comes from a holy root and a stock of truth (Israel) cannot be compared to one who comes from an evil stock (idol worshippers). Israel was purified at Mount Sinai and enters at once, in full completeness, into the holy covenant; for converts, the ancient impurity only clears gradually, sometimes over three generations, whereas Israel dwells in holiness immediately. A difficulty is raised on the verse 'as in the days of Noah' (rather than 'as the waters of Noah'), and it is explained: when the righteous multiply, the Holy One, blessed be He, rejoices and spreads peace in the world - one peace for the union of male and female, another for the blessing of the patriarchs. As long as a righteous person dwells in the world, judgments do not take hold, for he pleads for mercy on the world's behalf rather than seeking good for himself - as Moshe offered his own life for Israel ('blot me out of Your book') and did not relent until they were forgiven. Noah, by contrast, did not plead for mercy for the world when told 'the end of all flesh has come before Me,' concerning himself only with building the ark; therefore the flood is named for him - 'the waters of Noah.'