PRESS RELEASE: Letter from Steve Jobs
August 24, 2011–To the Apple Board of Directors and the Apple Community:
I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know. Unfortunately, that day has come.
Above quote from: http://blogs.wsj.com/deals/2011/08/24/steve-jobs-resigns-as-apple-ceo/
EDIT: Also this from Forbes: http://www.forbes.com/sites/briancaulfield/2011/08/24/steve-jobs-resigns-tim-cook-named-apple-chief-executive/