Here is his resignation letter.
Dear Mr. President: When the Supreme Court rises for the summer recess this year, I intend to retire from regular active service as a Justice, under the provisions of 28 US.C. § 371(b)(l), having attained the age and met the service requirements of subsection(c) of that section. I mean to continue to render substantial judicial service as an Associate Justice. Yours respectfully,
From:
http://www.scotusblog.com/wp-content/uploads/2009/05/dhsletter-2.pdf