I'm surprised no-one's mentioned his email to Mike Hearn on the 23rd of April 2011, where he said that he had "moved on to other things". It was a personal email, but it was the closest thing to a farewell he sent to anyone:
From: Satoshi Nakamoto email@example.com
Date: Sat, Apr 23, 2011 at 3:40 PM
To: Mike Hearn firstname.lastname@example.org
Subject: Re: ...