Said to be a remark made to his servant when a student asked what he would get out of studying geometry.
'threepence' renders τριώβολον "three-obol-piece". This amount increases the sarcasm of Euclid's reply, as it was the standard fee of a Dikastes for attending a court case (μίσθος δικαστικός), thus inverting the role of teacher and pupil to that of accused and juror.
The English translation is by The History of Greek Mathematics by Thomas Little Heath (1921), p. 357 http://books.google.com/books?id=h4JsAAAAMAAJ&pg=PA357#v=onepage&q&f=false. The quote is recorded by Stobaeus' Florilegium iv, 114 ( ed. Teubner 1856 http://www.archive.org/stream/iohannisstobaei00meingoog#page/n598/mode/2up, p. 205; see also here http://laudatortemporisacti.blogspot.ch/2011/04/anecdote-about-euclid.html). Stobaeus attributes the anecdote to Serenus.
Attributed