Charles Ellicott Commentary Deuteronomy 28:12

"Jehovah will open unto thee his good treasure the heavens, to give the rain of thy land in its season, and to bless all the work of thy hand: and thou shalt lend unto many nations, and thou shalt not borrow." — Deuteronomy 28:12 (ASV)

The LORD shall open to you his good treasure, the heaven to give the rain. —The Jews have a saying that, “There are three keys in the hand of the Holy One, blessed be He! which He has not entrusted to the hand of a messenger, and they are these: the key of the rains, the key of birth, and the key of the resurrection of the dead.” The key of the rain, as it is written (Deuteronomy 28:12), Jehovah shall open to you His good treasure, etc. (from the Talmudic treatise, Ta’anith, p. 20, b).