There are famous quotes about what Farcas Bolyai wrote to his son Janos to persuade him not to study the "theory of parallels " or what is now known as hyperbolic geometry
But not all translation of this quote are the same.
What was originally written (and for fun what is the most outrageous translation of it)
The wikiquote site gives the original quote from a letter of 4, April 1820:
A parallelákat azon az útan ne próbáld: tudom én azt az utat is mind végig — megmértem azt a feneketlen éjszakát én, és az életemnek minden világossága, minden öröme kialudt benne...
which it translates as
Do not try the parallels in that way: I know that way all along. I have measured that bottomless night, and all the light and all the joy of my life went out there.
(It reads like a Morrissey song lyric.)
The MacTutor site gives the same translation of the April 1820 letter, while also quoting an earlier letter as stating :
Detest it as lewd intercourse, it can deprive you of all your leisure, your health, your rest, and the whole happiness of your life.
In addition to these two quotes from two separate letters, the TodayInScience site quotes a third letter from 1820 stating :
For God’s sake, please give it up. Fear it no less than the sensual passion, because it, too, may take up all your time and deprive you of your health, peace of mind and happiness in life.
So we have three different letters, each expressing similar sentiments. Citations for the sources of each of the three translations can be found on the TodayInScience site (linked above).
Alternatively, this Hungarian site, devoted to the life and work of János, simply states :
In the spring of 1820 he informed his father about his experiences concerning parallels. Farkas in a long letter warned his son against this: “…don’t go any step further, or else you’re a lost person.”
EDIT
Rereading my answer over my morning coffee it seems clear to me that the second and third quotes are different translations of the same text. So it appears that we are dealing with two different letters expressing similar sentiments.
According to Jeremy Gray's " Jonas Bolayai, Non-Euclidean geometry and the nature of space" (2006) and "worlds out of nothing " (2007 ) it is all from just one long quote:
you must not attempt this approuch to the parallels. I know this way to the very end. I have transversed this bottomless night, which extinguished all light and joy of my life. I entreat you, leave the science of parallels alone [...] I thought I would sacrifice myself for the sake of truth. I was ready to become a martyr who would remove the flaw from geometry and return it purified to mankind. I accomplished monstrous, enormous labours: my creations are far better than those of others and yet I have not achieved complete satisfaction [...] I turned back when I saw no man can reach the bottom of this night. I turned back unconsoled , pitying myself and all mankind. Learn from my example: I wanted to know about parallels. I remain ignorant, this has taken all the flowers of my life and all my time from me.
And yet again:
I admit I expect nothing from the deviation of your lines. It seems to me that I have been in these regions; that I have travelled past all reefs of this infernal Dead Sea and have always come back with broken mast and torn sail. The ruin of my disposition and my fall date to this time. I thoughtlessly risked my life and happiness -aut Ceasar aut nihil (either Ceasar or Nothing).
in "Jonas Bolyai, Non-Euclidean geometry and the nature of space" (2006) the reference is to Stackel's "Wolfgang and Jonas Bolyai, geometrische untersuchungen, ... " Leipzig, Treubner 1913 page 81
In "worlds out of nothing " (2007 ) the reference is to Menschkowski's "Non euclidean geometry "New York, Academic press,1964 (tr. A Shenitzer) page 31-34
It would be nice to have a complete translation of what was written