I read some articles about Satoshi's history and stumbled upon this one:

No one knows what Satoshi is up to, but one of the last emails he sent to a software developer, dated April 23 2011, said “I’ve moved on to other things. It’s in good hands with Gavin and everyone.”

Source: http://www.coindesk.com/information/who-is-satoshi-nakamoto/

This sentence of Satoshi is quoted many times in the web, but I can't find a source who published it or to which developer it was send. I know there are archives of Satoshi's mails but I didn't find this statement in any of them.

Does anybody have further information about it?