Quote Without A Source
I would recommend against it. I've been bitten by this in the past: A famous quote attributed to a certain person, that after some investigation seems to be mistaken. 

Specifically: Many sources claim that Newton referred to "The Great Principle of Similitude" in regards to dimensional analysis, but having searched all through his Principia and other writings and not finding it, I'm now pretty sure the phrase comes from Rayleigh in a letter to Nature (No. 2368, Vol. 95; March 18, 1915). There's even a class at Stanford by that name, which recently changed its description to attribute Rayleigh instead (link). 

In summary: If you can't source it, then you probably shouldn't reference it. I feel pretty bad about saying something to several years of math students that turned out to be incorrect. Make a commitment to not distribute misinformation.