Babylonian Talmud, Ta'anit 7b

Rabbi Chama the son of Rabbi Chanina said: The day when rain falls is as great as the day on which heaven and earth were created, as the verse says (Isaiah 45:8), “Cause the heavens above to drip, and let the skies pour down righteousness; let the earth open, and let salvation and righteousness be fruitful; let it cause them to sprout together; I, the Lord, have created it.” ‘I created them [the heavens and earth]’ is not stated, but ‘I created it [the rain].’ [Artscroll and Judaica Press translation]