As quoted by Florian Cajori, A History of Mathematics (1893) p. 303, citing Franz Schmidt, "Aus dem Leben zweier ungarischer Mathematiker Johann und Wolfgang Bolyai von Bolya," Grunert's Archiv, 48:2, 1868.
Farkas Bolyai: Frases en inglés
Letter to János Bolyai (4 April 1820)
Published in: Samu Benkő (ed.), Bólyai-levelek, Kriterion, 1975, p. 123
As quoted in: O'Connor, John J.; Robertson, Edmund F., "Farkas Bolyai" http://www-groups.dcs.st-and.ac.uk/history/Biographies/Bolyai_Farkas.html, MacTutor History of Mathematics archive, University of St Andrews.
Having himself spent a lifetime unsuccessfully trying to prove Euclid's fifth postulate, Farkas discouraged his son János from any further attempt.