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.”
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.