Midrash su Salmi 83:2

R. Chama b. Chanina said: "The day of rain is of equal importance with the day on which heaven and earth were created; as it is said (Is. 46, 8) Drop down, ye heavens, from above and let the skies distill blessing; let the earth open, and let them all be fruitful of prosperity, and let righteousness spring up likewise I the Lord have created it; i.e., created them [the heaven and earth] is not said, but created it [referring to the rain]." R. Oshiya said: "The day of rain is so great that even prosperity is increased because of it; as it is written (Ib.) Let the earth open, and let them all be fruitful of prosperity." R. Tanchun b. Chanilai said: "Rain does not descend unless the sins of Israel are forgiven; as it is said (Ps. 83, 2) Thou hast been favorable, O Lord, unto thy land [with rain, only after]; thou hast brought back the captivity of Jacob. Thou hast forgiven the iniquity of thy people; thou hast covered over all their sin. Selah." Zeiri of Dehobath said to Rabina: "This ye learn from the above passage. We, however, infer it from the following: (I Kings 8, 34) Then hear Thou in heaven, and forgive the sin, etc."