Babylonian Talmud, Ta'anit, 6b

Rabbi Abahu said, “From when do we bless on the rain? From when the groom goes out toward the bride [an idiom for a large amount of rain]. What does one bless? Rav Judah said in the name of Rav: 'We give thanks to You, Hashem, our G-d for every single drop which you have caused to fall upon us.'”