'Know that joy is rarer more difficult and more beautiful than sadness...'

Quote:

'Know that joy is rarer more difficult and more beautiful than sadness once you make this all-important discovery you must embrace joy as a moral obligation.'

I heard this attributed to Gide. A search on-line turned up the quote but no reference to the source. I downloaded everything on gutenberg, can't find it. It should all be in the public domain; I'm surprised it isn't all on-line. Perhaps I need a French source.