It is not in Heaven
Part of a verse from the Bible, Deuteronomy 30:12, where it refers to God's law. Used in one of the most mind-blowing sections of the Talmud I can think of. It's in the Gemara, tractate Baba Metzia, chapter 4, page 59b.
The Talmud mentions the case of some particular type of oven (and debates what exactly its characteristics are, etc.) and brings down the events ensuing from a debate among the Rabbis over its ritual status. Such debates are very common; in fact, the Gemara mostly is the recording of these debates. In this case, the Sages (the majority of those assembled) held that this oven was susceptible to ritual impurity, while Rabbi Eliezer maintained that it was not. But he didn't concede in the face of the majority, as usually happens (by the way, this wouldn't mean that he was wrong, merely that the law followed the other opinion. It's not the same thing.) Picking up the story, what follows is a fairly close translation of the text.
Rabbi Eliezer brought all the proofs in the world to try to prove his point, but they were not accepted. Finally, he said, "If the law should follow me, let this carob tree prove it!" And the tree was uprooted from where it was and thrown 100 cubits away (some people say 400 cubits). They answered, "We don't bring proofs from carob trees."
"OK, then if the law is like me, let the aqueduct prove it!" And the water in the aqueduct started flowing uphill.
"We don't bring proofs from aqueducts."
"If I'm right, let the walls of this house of study prove it!" And the walls started shaking, as if to fall down.
Rabbi Yehoshua rebuked the walls: "You keep out of this! This is a debate among scholars, and no concern of yours!" So the walls didn't fall, out of respect to him, but they didn't stand up straight either, out of respect to Rabbi Eliezer, so they remained sort of leaning.
"If I'm right, let it be proven from Heaven!" And sure enough, a Heavenly Voice manifested itself and said, "Why are you giving Rabbi Eliezer such a hassle? He's right! In fact, he's right in all his arguments! The law should follow him!"
Rabbi Yehoshua stood up and said, "It is not in heaven."
What did he mean by this? Rabbi Yirmiah explains that it means that since the law was already given at Mount Sinai, we cannot take advice from a Heavenly Voice, since it already says in the law, to follow the rule of the majority (not necessarily the same translation as usual in English, but it's from Exodus 23). (In other words, now that God has given the law into our hands, it's in our hands. Not even he has the right to mess with the legal system he established: we can outvote God).
Epilogue: Rabbi Natan visited with Elijah the prophet, in Heaven, and asked him what God was doing when all this happened. Elijah answered that God smiled and said "My children have defeated me, my children have defeated me.
...And then things go on about the social impact, Rabbi Eliezer's excommunication (not quite the same thing in Judaism as it is in Christianity) for refusing to accept the rule of the council, the difficulties that ensued, because even though they were right to do it (to maintain the authority of the legal system) he was still in some sense "wronged," and God doesn't let such things go by easily... it gets into a different topic here. Which is really how it all came about in the first place, since the topic the Talmud was discussing was people being wronged. The story after this is interesting, but not quite on-topic
This really blows my mind. The whole business of "we can outvote God" is a pretty incredible message. It's a view of divinity that I think is quite unlike that of Christianity, but on the other hand it's a view that considers that God could vest his authority in humanity and trust them enough to make it theirs and not his.
Thought you folks might find it interesting.