source of "A designer knows he’s achieved perfection..."

[Darkreader]

There is a famous quote “A designer knows he’s achieved perfection not when there is nothing left to add, but when there is nothing left to take away”, it was even used in one of the Civilization games.

However I tried to find its source and all I can find is that it was said by Antoine de Saint-Exupery - but where? In which book? Do you know anything about it?

PS is there a way to create topic about the author and not about the book?