Watch Billy Crystal Give David Letterman a Hilarious Musical Send-Off

David Letterman still has over a month left in his historic late-night run, but Late Show favorite Billy Crystal gave his friend an early send-off on Tuesday's episode. Borrowing the tune from Fiddler on the Roof's "Sunrise Sunset," the comic-actor sang a nostalgic ballad about Letterman's 33-year tenure, as iconic images from the show flash on the screen. Crystal tells Letterman in the clip. "You're my best friend on television, And what I can't put into words I'd like to put into a song."