Maybe you mean the statement, “I believe because it is absurd,” by the Christian theologian Tertullian, of blessed memory

Thank you very much!

Afterward I searched Google using various phrasings,
and I found in Rabbi Nachman’s Conversations (225), as follows: “And he said that where the wisdom of philosophy ends — there begins the wisdom of truth, which is the wisdom of Kabbalah. That is: for the philosophers investigated only up to the spheres, and from there upward they know nothing at all. And even in the areas of wisdom from the spheres and below, they too are very confused in most, if not all, matters, as they themselves know. And the wisdom of Kabbalah begins where their wisdom ends, namely from the spheres and upward. For the entire ‘World of Action,’ including the spheres, the wisdom of Kabbalah includes in one word, namely the World of Action. And all the wisdom of Kabbalah deals with Action and above it, namely Formation, Creation, and Emanation, higher and higher, etc. And even within Action itself, in the inwardness of Action, that is, the spirituality of Action, the philosophers have no knowledge at all. But the wisdom of Kabbalah speaks only from the root of Action in spirituality and upward. It follows that where the wisdom of the investigators ends, from there the wisdom of Kabbalah begins.”

Of course, Rabbi Nathan’s words do not mean what people use this sentence to mean… his intention is to say that Kabbalah is above philosophy.

But I found the exact wording in a song by Uzi Hitman — “A Light unto the Nations” (lyrics on Shironet), as follows:
Where reason ends / faith begins / and there are things we will not be able to prove / even in another million years.

Because this sentence is used as a very handy axiom in arguments about faith, it was important to me to know its source, so as to reject its use.
I’d be glad if anyone knows real sources for this sentence.

You can — and should — reject its use even without knowing its source.
And if you mean that if its source is Christian you’ll be able to get people to stop using it, then I’m sorry to disappoint you: they won’t stop. Their reason has already run out.

The Breslover definitely said it:

“The main thing and the foundation, upon which everything depends, is to bind oneself to the righteous man of the generation, and to accept his words regarding everything he says, whether a small matter or a great one. And not to deviate, Heaven forbid, from his words to the right or to the left, as our Rabbis of blessed memory said (Sifrei, Parashat Shoftim): even if he tells you that right is left, etc. And to cast away from oneself all forms of wisdom, and remove one’s own understanding as if one has no intellect at all apart from what he receives from the righteous man and the rabbi of the generation. And as long as any independent intellect remains with him, he is not complete and is not bound to the righteous man.”
Likutei Moharan 123:1

“He said: Whoever will obey me and fulfill everything that I command will certainly be a great righteous man, no matter what. And the main thing is to cast away one’s own intellect completely, and simply fulfill everything according to what he says. And he then mentioned the matter of ‘a disgraceful people and not wise,’ as explained in Likutei I, section 123; presumably, whoever is able to learn is the more fit.”
Chayei Moharan 320:1

https://www.sefaria.org.il/Likutei_Moharan.123.1.1?lang=bi&with=all&lang2=en
https://www.sefaria.org.il/Chayei_Moharan.320.1?lang=bi&with=all&lang2=en

Dear Not a Frummer, with all due respect, you did not understand what I was asking.
I was looking specifically for a source for the wording of that sentence itself, not whether something similar appears in Rabbi Nachman’s writings.
Thank you very much.

Dear Mr. Israel, I understood. If so, I think the source you are looking for is in the writings of the Christian philosopher Søren Kierkegaard, specifically in his book Fear and Trembling.

" because faith begins precisely there where thinking leaves off."

Click to access 101.%20Fear%20and%20Trembling%20book%20Kierkegaard.pdf

Hope this time I was of help to you.

Much appreciated!

After further searching in various phrasings, I found an authoritative Jewish source for this statement (I am not discussing the substance of the statement, only its source): Rabbi Shach, of blessed memory, brought in the name of Rabbi Yitzchak Zev Soloveitchik, who said in the name of Rabbi Chaim (Avi Ezri I, p. 41; on Maimonides, Laws of Repentance 5:5): “Now, I had always wondered why this commandment is called faith, for simply speaking it is among the first intelligibles, since the world cannot exist without a leader, and ‘from my own flesh I behold God’; and the wondrous wisdom in every tiny creature is beyond estimation, all the more so the wisdom in the creation of man — so how is it possible that all this exists without a leader? Yet Maimonides there wrote ‘to know God’ — this is knowledge. And in Scripture, in the enumeration of the commandments as commandment 1, he wrote: it is the command with which He commanded us regarding belief in God.
And I asked this of our master, the great gaon Rabbi Yitzchak Zev Soloveitchik, of blessed and holy memory, and he told me that he too had considered this and asked his father, Rabbi Chaim, of blessed memory, who told him that certainly, to the extent that the human intellect can reach, it is something intelligible and this is not faith but knowledge; but the obligation of faith begins at the point where human reason comes to an end. He explained the matter on his own as follows: human intellect is limited by time and place, and whatever is beyond time and place is already outside its grasp; and the essence of the Holy One, blessed be He, is beyond time and place, where human intellect has no standing — and there the duty of faith takes effect.” End quote.
He also referred to Beit HaLevi (end of Parashat Bo), who similarly wrote:
“And another reason for this is that the essence of the commandment of faith is that we believe in all the words of the Torah, and according to the tradition from our Rabbis, also in that which we cannot establish by conclusive proof. For whatever has been established to a person by proof is called knowledge, and is not yet included within faith. But we are commanded to believe, and this applies to things that the intellect cannot grasp through proof. So long as one believes only what has been established for him by proof, he has no share in the commandment of faith at all.”

Many thanks to all those who helped and assisted!