According to CNN:
"Letterman announced his plan during a taping of "The Late Show" on Thursday afternoon. CBS is expected to issue a statement sometime before the program is broadcast Thursday night. A CBS spokesman declined to comment. Letterman's announcement initially leaked out when a performer, Mike Mills, who was at the taping of "The Late Show" tweeted out the news. "Dave just announced his retirement," Mills wrote. What exactly Letterman says about his decision on "The Late Show" is not yet known."