It comes from the end of the book, when the prince has left and all the pilot has to look at is the star. In English:
"Here, then, is a great mystery. For you who also love the little prince, and for me, nothing in the universe can be the same if somewhere, we do not know where, a sheep that we never saw has--yes or no?--eaten a rose . . .
"Look up at the sky. Ask yourselves: is it yes or no? Has the sheep eaten the flower? And you will see how everything changes . . .
"And no grown-up will ever understand that this is a matter of so much importance!"
I got it to remind myself - and anyone who sees it, and asks me about it - to always look deeper.