One section of the Tanya is called Likutei Amarim. Chapter 11 of this section contains the statement “Concerning such a person, the Rabbis of blessed memory have said, “The wicked are full of remorse.”
In the Bilingual edition of the Tanya, this sentence appears on p. 45. In the paper edition (but not in the online edition) it is marked with superscript 10. The related footnote 10 says “Nedarim 9b.”
In Lessons in Tanya, this sentence appears in volume 1 on p. 166, and is marked with superscript 8. The related footnote 8 says “Nedarim 9b. (So cited in early sources, though not to be found in current editions of the Talmud).”
This latter footnote is correct, at least as to the parenthetical part. Nedarim 9b does not contain the statement that the wicked are full of remorse.
I searched at Hebrew Books for the statement that the wicked are full of remorse, and the only match I could find was the book Sadeh Tsoifim, which appears to be a commentary on the Talmud, and which contains the magic words:
I’m not totally clear on the meaning of that remark, it being abbreviated right out of my ken.
At my shul we have a green dictionary of statements of Chazal, a dictionary which, unfortunately, I cannot describe more precisely right now. But I looked up the statement about the wicked, and the entry was almost exactly the footnote in Lessons in Tanya. It said that the statement was said to appear on Nedarim 9b in the book Shevet Mussar, but that it does not actually appear in the Talmud. Apparently, the book Shevet Mussar was written by the year 1729, the year when the author died, so (to paraphrase Dean Vernon Wormer) we know that he didn't copy from the Tanya.
Can you explain this? Are there, or were there, editions of the Talmud containing the statement? Did it appear on p. 9 of Nedarim? The Tanya was published in 1796, so (to judge from this Wikipedia article) the author might have had a printed Talmud such as the Bomberg Talmud of 1523 or the Benveniste Talmud of 1645, but probably not the Slavuta Talmud of 1795 and certainly not the Vilna Talmud of 1835. (But of course he doesn’t actually say that the statement is in the Talmud, exactly.)
Obviously we must all rely on books to tell us what the Rabbis said. But doesn't it seem strange that an author would attribute this expression to the Rabbis with no pre-18th-Century source for that attribution?